that's why, that's it
Yain usually comes at the end or middle of a sentence in contrast with kabay and inya which are mostly at the beginning.
Mapagmatag-ay yay na ta nakapag-aral yay na, yain.
He is proud now that he has become rich, that's why.
Mapagmahlay kay na kali ta nibande kay na, yain.
You are proud now because you have become rich, that's it.