Why do our pupils dilate when we look at or think about something we like?

I know its part of light entering more into your eyes that expands your pupil.
Source: my eye doctor

excitment causes activation of the sympathetic nervous system , which among other things causes pupil dilation

Also, the pupils constrict when they are looking at something close-up (accommodation), so if he has been looking at something closer, like a toy, and then looks up at you, they may dilate.

it also happens when you look at someone you fancy.

so you can get a better look at the things you like most :)