Why the heart often work against the head?

heart=emotions
head=intellect

And the head should win every time if the heart is telling you to be with someone who is harming you for example.

In less serous cases the head should factor in the heart before taking any actions.

Depends on the situation. There are times you should follow your heart. But hopefully, you have thought it out.

right intention?
right effort?
right view of the situation?
right action?
right effort?
right speech?
right livlihood?
right concentration?

If there is perfect balance (unattainable) in a person's life, there's no separation between heart and head. All we can do is to strive for a better balance between intellectual decision making and emotional decision making.

The terms head and heart are reflections of self...it is all one and to separate them, even for the sake of a discussion, can lead a person astray. The balance of emotional and intellectual is very complicated.

But love can screw your head up . . .
makes ya wanna write bad checks
and buy sexy underwear.