How can i make myself less selfish?

Whenever my friends are talking i tend to interrupt them... I dont mean to do this but its quite a nasty habit. My friend was talking and when she was done i spoke up about how i was getting new converse, since my other pair was getting dirty. Shes getting converse as well, but she kind of copied me with that. I was trying to be pleasant about it and not freak out. I wanted to wear our converse on the same day since they hadnt come in yet. I was in the middle of telling her they were coming in on wednesday and she called me a selfish b***** who burst other peoples happiness. Why am i supposed to do about my best friend?