Why do i feel used sometimes?

I don't have a lot of friends but one thing i've noticed in life and that is that people tend to use me. They only talk to me or associate with me to get something from me and when i call them on it, it will just create problems and so instead of acting on it i do the opposite. i distance myself from those ppl by not communicating with them and when i do this they also stop communicating with me.

About a year ago i met this girl in a class and we became friends. Platonic, nothing is there. Anyway, i start giving her rides to school this past semester, we had 3 classes together. She would invite me over for dinner or something every so often...once a month or whatever...her son is like 8 years younger than me and her daughter 10 years younger so we'd hang out a lil or whatever..

this next semester starts in january and i have 2 classes with her and 1 with her son which i intended on carpooling. they dont have reliable transportation like i do, they ride the bus to school etc...so now that its break from school and stuff they dont talk to me at all or anything or make any effort like how i felt i did. obviously i feel used for my vehicle...i'll not communicate with them and then if they hit me up for rides right before school starts, i'll end up making up some reason as to why i cant anymore...and so goes the story of my life. why are ppl likes this, or maybe why do i feel used constantly like in this situation