A "pig head" is a "stupid" brain.

The word "stupid" here can have n meanings, such as not knowing what, not knowing what, not understanding what, and so on.

Here is a deduction formula for college students and pigs:

College students = eating+sleeping+falling in love

Pig = eat+sleep

So: college students = pigs+love.

The above college students-falling in love = pig

That is to say, it is pigs that college students don't fall in love with.