Could you please help me understand this, especially the yellow part?

A- Why does it take a woman with PMT 20 minutes to iron a shirt?

B- Because it just does.


A - is that PMS?

B - that's the way it is.

Why do you love her? I don't know; I just do.

Why did he ever say that? I don't know; he just did.
It made me laugh.

"PMT", or pre-menstrual tension, is a condition related to the female menstrual cycle that is popularly supposed to make women irritable (amongst other things). The joke is that "it just does", if said in the appropriate tone, is an irritable response that one can give when one isn't in the mood to explain oneself. "It just DOES. ALRIGHT?!" That's how I understand it, anyway.