Yes, friendship can be lost after it's gained, and friendship can be reforged (to a point, see below) after it's lost.
Getting to +100 does not mean you're safe and can say anything you want around your companion. If you say something disagreeable, your companion's approval will go down.1
Getting to -100 puts your companion into crisis mode, and you get one chance to fix it through a dialog:
- If you navigate correctly, you'll gain approval and your companion will stay.
- If you don't, your companion will leave for good.
Similarly, if your companion enters crisis mode again by hitting -100 a second time, that's it: they'll automatically leave.
Note that this was "fixed" in Dragon Age II: companions won't leave (-100 just corresponds to "rivalry" instead of "crisis" or "hatred"), and once you hit 100 friendship or rivalry, it'll stay locked for the rest of the game.
Note 1: this is in part why you don't need +100 to initiate a romance: just +80 to +90 (depending on the companion).