How did I become so skeptical of nice behavior?

I don’t know how to accept nice behavior with grace. If someone randomly starts a conversation with me, extends a gesture, or even gives me their time, energy, and attention, then I meet that with skepticism. As in, I think that people only treat me that way because they want something from me, or they want me to do something for them. This is a skill that is necessary in life, but I think I employ it at the wrong moments. Openly interpreting friendly behavior with skepticism and questioning why they are being friendly with me in the first place is bad. It undermines the ideal point of friendly behavior, which is to express emotional connection with someone. It is also quite disrespectful. Let me explain…

If someone is being nice to you - the ideal kind of nice - it means they like you and they want to express that emotion. No transaction involved, no resource to gain, no service to be had; they merely want to show you they feel something for you, that they care. Meeting this intention with skepticism shows that you don’t believe that they care about you, and that they are doing that because of the aforementioned reasons; some ulterior motive. Behaving with this claim in mind makes people feel like they are being disingenuous and shallow, and because you - the recipient of kindness - are making them feel this way, you are calling them disingenuous and shallow by association. Obviously, implying this about people is, frankly, disrespectful and insulting. 

It makes some sense. This is assuming the skeptical person receiving kindness likes people being kind to them, of course. Even then, this assumption goes beyond being reasonable. Kindness is ubiquitously accepted among intelligent organisms, even if it might take more work than what this fact might make you expect. Except, there are also humans out there who are so damaged that they are incapable of accepting kindness ideally. Those kinds of people require more patience and grace than anyone. I am not sure if I am at that point… But, it’s like Deleno said: having this kind of skepticism, while uncommon, is not a bad thing itself. Acting on these thoughts of skepticism is where I went wrong, and I did not have anyone to tell me that doing so is wrong. Just pain, ego, and crushed friendships and relationships. In fact, I rescind my uncertainty of if I am that damaged. I am not. I just did the wrong thing and poached kindness from my friends while expecting some kind of transaction to occur. I simply could not see the value in the emotional connection by itself in all of the relationships I’ve ever had, which is probably even more disrespectful. Devaluing someone’s emotions hurts, and I should know that better than anyone. So, how did I become skeptical of nice behavior?