Because we are curious individuals by nature and tend to dwell on things over and over and over instead of accepting the change and living in the moment. Also it means you are human and have feelings! Don't try to shirk them, just work through them. Good luck.
If you are in love it takes a while but some ladies move on but others do not
Residual feelings you need to reconcile and resolve.
as long as you let it
You still like them for the moments you shared :)