Why am I always the rebound guy?

i've already been in a rebound situation twice already. Then just this week one of my friends who got out of a 2 year relationship a month ago decided to tell me she liked me and wanted to be more then friends. I learned from the first two that rebounds aren't good ideas, but this one is different because she ended the relationship. I really don't know what to tell her, and why am I always the rebound?
Why am I always the rebound guy?
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