❝If you're nice to others just to make yourself feel better. Isn't that essentially for selfish means?❞
You pose a question that seems straightforward, but I don't think it can be. The motive is described by you as "just to make yourself feel better', which supposes that there is no other motive. Rarely can an individual's motivation be so one-dimensional. I suspect that some people will feel a glow of satisfaction in being nice to others, but that does not mean that there is no aspect of that niceness being for other purposes such as wishing to relieve other people's emotional distress, or creating a pleasant environment for others, and so on.
If someone's sole motivation is to make oneself feel better, I would still hesitate to call it selfish. For instance, that person may have had poor self-esteem for some time, and is now learning to consider others. For some time, the sole impetus may be to improve one's own mental health or one's own sense of worth or "making one's soul". Another example may be when someone is in recovery from alcohol dependency or drug dependency, where part of the process is to acknowledge one's own faults, admit those and ask those one has hurt for forgiveness, and paintfully putting together a new way of dealing with other people. At first, it's not altruistic. However, altruisim can be learned after changing one's behaviour. The mind changing as a result of changed behaviour is one valid way... It might even be more usual than behaviour changing as a result of a changed mind. I can only applaud that. I wouldn't call it selfish.
If someone's "niceness" is a façade only, where the charming demeanour cloaks unpleasant thoughts and secretive acts against others, well! That's another thing altogether. It's without question that we can deceive ourselves and have a very different sense of oneself as opposed to how some may see us... Generally speaking, people cloaking selfishness with a charming pleasant mask will fool some, but not others. Yes, that sort of "niceness" is likely to be primarily or essentially for selfish means. Perhaps if one delves into that person's behaviour more closely still, one might see something other than selfishness at work there, but that's probably a matter for a psychologist.
We can't include psychopaths in this point, because as I understand it, making themselves feel better is not part of how they operate. Making themselves feel powerful or making it possible to manipulate would fit, but that's not how you phrased your question.
❝Can you be "too" nice? and how much is not enough?❞
I assume that you are thinking of "too nice" as being the same as trying to please everyone, or not challenging any bad behaviour - in essence, being a wimp. If so, I would agree. One can simply not please everyone, and it's emotionally exhausting trying to. Never challenging anyone's behaviour makes one a potential victim of emotional and even physical abuse (but of course it depends on many other factors). One might fail to discipline a child out of being "too nice" aka no challenging of bad behaviour and enforcing of rules. One might fail to act when someone else is being a bully (aka non-acting bystander who by his/her very failure to act is unawares helping to create an environment in which the bully can flourish). And so on.
However, if we rule our this sort of failure to act, and leave niceness to function on its own terms, perhaps it isn't possible to be "too nice". Niceness does not equate by definition to folly and it doesn't mean the same thing as overlooking wrong behaviour.
In terms of not enough niceness... hmm. Is it possible to define that rigorously? I know that there are people I think of as unpleasant. I don't have a strict set of rules to define that in every circumstance; however, my gut pipes up when someone is being un-nice. I do dislike behaviour that is deliberately unkind and offensive.
If you mean behaviour that is too saccharine, that's another thing. That isn't niceness.
❝Would you trust someone that's always overly nice to you for no apparent reason?❞
If my gut sends me warnings about the genuineness of that niceness, I would be very cautious. Fake niceness (when I sense that there is no genuine liking or respect for me) is intrinsically untrustworthy. I prefer to take people at face value, but admittedly sometimes my bovine ordure meter goes PING! and there we are.
❝Are you a "nice" person? What do you do that makes you nice?❞
Yes, in my own estimation of myself, I am a nice person. Some may disagree. I want to be of help to others rather than destructive towards others, which I think is the grit and kernel of niceness. I certainly feel better (emotionally and physically) if I act kindly, which leaves me open to the accusation of being a selfish do-gooding inflictor of niceness for my own benefit to those who think of niceness / kindness in that way. Hence I will end this reply with a cream pie in the face, just to knock you off balance a little...
For that, you get another pie in the face!
And another.
I could do this ad infinitum. There is something so very satisfying (without losing my claim to niceness) about a virtual cream pie in the face. I'm providing you with dessert, too.
