A lot of it is self contentment, however a lot of it is also allowing yourself the freedom to be yourself and giving yourself permission to say 'no' when you want to say no. There is nothing wrong with being nice, but when you are sacrificing your own personal freedom and happiness in order to do so, then you are only hurting yourself, and others will see that and take advantage of it.
In my experience, I've found that people tend to gravitate towards those who can boldly be themselves, no matter what it is that makes them so. While they tend to say that opposites attract, I've found the opposite true when it comes to human interaction and grouping, in the sense that a kind hearted person will attract more kind hearts into their lives, and someone who is a jerk will attract more jerks into their life. Seeing as how you are a kind person who is always willing to help out, I have no doubt that, should you free yourself, you would attract some wonderful people into your life. Though it doesn't happen overnight. Sometimes the best people take even years to discover or to find their way into our lives. But it's these people who will make us feel alive, and feel good about ourselves.
So, to conclude, I'd like to advise you to grant yourself the freedom to be yourself, whether that means making silly/strange sounds throughout the day, randomly bursting into song, or even sporadically dancing to a beat, whether it be invisible or clear as day. Also, don't be afraid to hurt someone's feelings if what you feel is not in alignment with what they would like. Grant yourself the freedom to say no to things you disagree with, and to say yes to the things that you do agree with. When you set yourself free this way, like minded individuals will be able to identify themselves in you, and through that you will have a friend in others. Not only that, but you will also feel good about being who you are, and that comes with a sense of personal happiness and contentment as well. It's a road to a better, happier you, and there is no better time than now to start exercising that kind of freedom. You'll be quite thankful you made the decision to do so, I can guarantee that.