Why does my mouse want to live with my rats so bad?!?

Rats are very social and accepting animals. A territory war is more likely to happen if the rats were to invade the mice's territory. Some animals just don't have that fighting instinct ie. cats and dogs that get along. I had a mouse that lived in the cage with my rat...they were great friends. As for reinforcing the cage door. I once used one of those little diary locks...the ones with the generic key. It worked just fine.