Zenkichi loves Medaka because he's been 'by her side' since they were little kids and because he's attracted to her. It's that simple.
Kumagawa loves her because Kumagawa wants to break her and her ideals. Kumagawa's love is twisted, like everything else about him.
If Zenkichi doesn't end up with Medaka I guarantee you he won't end up with anyone. Shipping is bad and you should stop doing it.
