You have in a way answered your own question. You push people away and cut the conversation short. Regarding why you are like that, no one can answer that except you. If you don't have an answer, try to find it. Others can just guess and guessing is not good when it comes to assessment of your personality. You know yourself more than anyone else does.

I didn't make many friends in college but I am lucky I do have 3 people that I meet in college and we have become life long friends and we graduated almost 50 years ago. One lives in Pennsylvania, one in Vermont and one in Illinois.

Gee, I wonder why? You seem like such a fun person...🙄

may be none of them is worth your time