Dwelling in mountains, caves, even in isolated rocks, these creatures of Norse and Scandinavian folklore, are rarely helpful to human beings. Most often nightmarish in physical description-- they can be hostile and mischievous.
It is a belief that lightning frightens trolls away and that the din of church-bells causes trolls to leave for other lands-- this is meant to explain the absence of trolls in many of the Scandinavian regions. Then there are the Stone-Trolls, like the ones from Tolkien's The Hobbit, who turn to stone in sunlight.
From childhood, I can remember hearing/reading the old fairytale, Three Billy Goats Gruff, with the goat-brothers trying to cross the bridge to greener fields. Only, they have to contend with the Troll that lives under the bridge--- he didn't ask them to cross his bridge, he didn't seek them out... I always felt a bit sorry for the Troll, who was only protecting his bridge-home--- and perhaps looking for a tasty meal.
Beware the Path You Take
Beware the path you take,
especially one you've never walked before,
you never know who's waiting there,
and what, for you, they have in store.
Cross one bridge,but don't look down,
cross the next one just the same,
don't tarry long when the growl you hear,
sounds strangely like your name.
Don't let them catch you as you pass,
the price you'll pay, a heavy toll,
rush fast beyond the creaking bridge,
wake not, the sleeping troll.
--- e.a.s. demers