For me, first of all Topcoder is all about interesting problems.
I am the ACM ICPC 2014 World Champion
Solve problems, and love solving problems.
I started participating in programming contests in 2008. Topcoder had (and it still has) one of the best regular online contests. I saw my friends competing, and joined them. I'm competing only in the Algorithm track. After two ACM ICPC years I had a small break, but competing in TCO showed I still remember something.