Arbitrariness isn't the issue. Agreement between various moral codes -- often in harmony, often conflicting -- is what matters. And often that means compromising on some things. You don't appear willing to compromise on what seems like an advocation for absolute property ownership rights.
What other way is there to live than to treat others neither as masters nor slaves, but as trading partners (in love, friendship, mentorship, business)?
You're advocating the opposite of that. As a landlord -- assuming a world with absolute property rights -- you have a significant power advantage over your tenant. A 3x-4x rent increase is not treating your tenant as a "trading partner", it's using your status as landowner to your enrichment and their detriment.
Further up you suggest:
A group that votes to dictate what I can do with my property might be doing it by the letter of the law, but that doesn't make it morally right
Not inherently, no. That's only the case if you believe that the voting-majority group is immoral. That certainly can be the case, but I don't believe it's the case often enough to abandon the idea of democracy.
Personally, I'm definitely in favor of limiting property rights. Property is in general not a fungible "product", and its supply is limited. I consider land to be a public good. The state grants you some property rights, but reserves to place some restrictions on what you can do with the property in the name of the public good. I don't think we have the balance down perfectly yet, but I would absolutely not advocate giving property owners carte blanche to do whatever they want with that property.
It's a matter of drawing the line somewhere: I think you might agree that some restrictions are good. Disallowing renting decisions based on discrimination of protected classes is something we might agree is a good thing (sorry, I don't mean to state your position for you, but I don't know you, so I'm just stating a hope, a guess, as to somewhere we might find common ground). If we can take that as a given, then we can agree that the state restricting property owners to some extent does indeed make sense. And from there it's finding a list of things that it's ok to restrict, and it's not ok to restrict. From this discussion I'd guess that my list of things that it's ok to restrict is a bit longer than yours. And that's ok, but at least it's a starting point from which to find a compromise. The final list has more on it than you want, and less on it than I want.
Business deals and compromise are rarely about making one or the other party (or both) 100% happy. I believe there's "common wisdom" floating around that the best deal is one that both sides are slightly uncomfortable with, and I believe that to be pretty true.
If, on the other hand, you do truly believe in giving property owners absolute power and free reign to do whatever they want with it, then I don't know that we can actually have a productive discussion, because our viewpoints may just be too different. That's unfortunate, but that happens sometimes.
A 3x-4x rent increase is not treating your tenant as a "trading partner", it's using your status as landowner to your enrichment and their detriment.
In a romantic relationship, either party can decide to leave without retribution.
In a friendship, either party can prioritize something else they value more, and the friendship wanes.
Students can outgrow mentors, and vice versa. Businesses can fire clients, and clients can go to other busiensses.
It can definitely be detrimental for someone's romantic partner or spouse to leave them (and the same can be said for any of the other relationships in this list). Some people are so distraught when someone leaves them that they attempt suicide.
However, does the fact that this happens in any way justify imposing restrictions on couples? Should a person be forced to endure a romantic relationship that they wish to end because it would be to the detriment of the other person? You can say the same thing about any of the relationships I've listed. For example, should you be forced to buy a brand of soap you don't like because the grocer isn't doing well, and he needs you to buy his unsold inventory?
All relationships are a two-way street, and for a fair trade, both parties have to agree, voluntarily.
These are all just specific cases where one can reasonably make arguments to say "yes" or "no", but have a different opinion for other specific cases.
For your first 3 examples (romantic, friendship, student/mentor), I of course say sure, either party is free to leave whenever they want.
The difference here is an economic one. It's near impossible to make sensible legislation around feelings (though some lawmakers certainly seem to try). While the ending of a romantic relationship may have a huge negative psychological impact on me, it doesn't affect my ability to live on in economic terms. A landlord kicking me out of my apartment can directly affect my livelihood, and the government is trying to protect me there. I think it's also uncommon for that to hurt the landlord (especially in the non-rent-controlled case), so there's an imbalance of pain. In the relationship case, leaving causes me pain, but staying causes the other person pain. Legislating whose pain is worse or less justifiable is, I hope we'd agree, a mess.
The firing topic is interesting, though, because even the law disagrees in different parts of the world. In Europe, for example, many countries have laws that are very employee-centric, and make it very difficult for firms to fire people. I personally don't agree with the extent that they've gone over there, but I can at least appreciate that people in other areas value employee security over employer mobility and lack of overhead. Even different states in the US have different laws that make it easier or harder to fire people, and define different obligations on the employer when they want to fire someone.
All relationships are a two-way street, and for a fair trade, both parties have to agree, voluntarily.
I'd like to agree to that in principle, but that just isn't true. There are plenty of laws on the books that can give one side more rights to make, not make, continue, or terminate an agreement. For example, a landlord may wish to discriminate against people with certain physical disabilities, because otherwise he'll have to spend money to install accessibility features in his building, but the law says... nope, can't do that.
A landlord may also wish to discriminate against an existing tenant who isn't able to fulfill the landlord's newest vision of his property's profit-making abilities, but there are a bunch of laws that also say... nope, can't do that. I happen to agree with a lot of them, or at least the intent behind them.
Our entire legal system is based on the idea that, on an individual level, both parties don't have to agree... only some representatives pulled from the majority do... sorta. There are lots of laws on the books that I never agreed to, and yet I'm bound to follow them just the same.
But all that is fine, I think. Many laws -- renting-related laws in particular -- are designed to weaken inherent power imbalances. The landlord naturally has more power in society than the single mother who is holding down two jobs. The law tries -- often imperfectly -- to protect the mom from people who would ignore her interests and take advantage of her.
