I don't know Finnish, but I like exploring it. While having a look al kinds of derivations based on 'full', I came across täytyä, which seems to mean something like 'have to'. I Just wondered whether there could be an etymological link with täytää (or ...), filling. Maybe not at all (I am very good at wishful thinking), but on the other hand: when something (...) is full, some kind of urge might arise. That is what I thought. Could that make sense?