Or, hear me out, their teeth have to grow because they like to chew so much they'd wear them down and starve

Really a mouth half full kinda of guy.

Unfun pet rat fact: if their teeth start growing at weird angles for whatever reason, this mechanism stops working and you have to get the teeth trimmed every couple weeks.

> If they don't wear down their teeth, they'll grow out of control and kill them.

cf the Babirusa - "If a male babirusa does not grind his tusks (achievable through regular activity), they can eventually keep growing and, rarely, penetrate the individual's skull."[0]

[0] https://en.wikipedia.org/wiki/Babirusa

To be fair, the biological mechanism for motivating behaviors is usually by making it rewarding.

They both have to and like to!