I am one of those people who places a sentimental or emotional anchor on a possession. Therefore it is risky for me to keep old stuff, as it slides me back to the feelings I have associated it with which is usually bad because of a failed relationship.
If I am in a new relationship I dont see the point of keeping them as it distracts me from committing to the present situation. I usually have the same reaction to other people keeping old stuff in new relationships. It reminds me of gfs hanging out with their bfs and glancing back at the person who walked past.