What do you mean by "doesn't have to"? Do you mean it's going into overdrive at too low a speed?

You can be cruising at 45mph and it will just shift into overdrive. No hills, no passing anyone, it just shifts.

Doesn't do it all the time either.

It should be in overdrive before 45 mph if you're cruising at a steady speed. If all the other shift points are correct, the governor valve is working properly and the kick down cable or linkage is adjusted correctly. If it shifts into overdrive too soon when you're still accelerating, that is an issue with the kick down adjustment. Once they switched from using a linkage to a cable, sticking cables became common and would cause delayed up-shifts, and down-shifts that happened too soon before the vehicle speed got low enough.