There is a very fine line between being nice and being a pushover, a very fine line.
Yes, most of us are inclined to be nice and maybe even go out of our way for some people from time to time. But, if you find yourself doing favors all the time or to the point that your own life is being affected by it, you've become a pushover. I've learned this is especially true if you have roommates you're not necessarily friends with, but have to cohabitate with and would rather there not be any sort of tension.You are not obligated to take time out of your life and inconvenience yourself for others, especially when those people wouldn't do the same for you. If you're busy, say you're busy, or just say no. It's ok to say no.
I know how this feeling because I was one of the biggest pushovers out there. I would go out of my way for people I had just met, a lot of the times at the expense of my own convenience. I would do things for others, knowing damn well that if the roles were reversed they wouldn't go out of their way like that for me. I was worried about the confrontation that could arise if I said "no" and it wasn't received well by the other person.
But sometimes you have to take a step back and just say "who cares." Who cares if they're upset or annoyed that you don't want to take them to the store or whatever; you are not responsible for them.
I came to a realization one day that I needed to start putting myself first and say "no" to people. I still struggle with giving people a flat out "no" but I'm working toward it. It's a process. Since I've started to say no, life has been so much better. That's not to say I'm not a nice person and don't do anything for anyone anymore, but I am more conscious. If I'm busy, I'll say "I'm busy," and it doesn't have to be something consuming your every second for you to claim that you're busy. Maybe you're in the middle of reading an article or scrolling through your news feeds. You don't owe anyone an explanation. If you don't want to do something, don't do it.
It's important for all of us to worry about ourselves first and not feel bad about it. Seriously, you owe it to yourself.