I use to do that a lot! I could never say no and even now I hate refusing people but I know that if I don't, people are more likely to push me around. I learned my "say yes" behavior from my mom. She's really sweet and willing to help anyone and everyone even when she's crushed under a rock. I kind of learn to be more assertive through her mistakes and because when I went to low-minority schools I had to adapt the mentality that I'm "unique". Pretty much faked it till I made it and I miraculously gain confidence, well a little bit.