Little is known about the night elves' precise origins, for their race was formed so long ago that no hard evidence has surfaced to prove or invalidate this theory. All that is known for certain is that a tribe of nocturnal humanoids came to live on the shores of the first Well of Eternity, and the Well's cosmic energies changed them into the night elves we know today.
Certainly many trolls do believe that the humanoids who developed into the night elf race were trolls. The theory does have some credibility, for there is at least a superficial physical resemblance between trolls and night elves. Furthermore, the troll race dominated much of ancient Kalimdor--the only continent on Azeroth before the Great Sundering--long before the night elves came into power.
Nevertheless, many night elves find this theory preposterous and abhorrent. They are quick to point out that the first night elves began their rise to power by defeating a number of nearby troll tribes. As a consequence, the trolls came to fear and respect the might of their new rivals. The troll theory of night elf ancestry may have been a direct result of this early conflict. The trolls hated the night elves--a sentiment that persists to this day--and may have wished to marginalize the night elf race and its accomplishments. Also, attributing the night elf race with a troll heritage likely helped the trolls come to terms with their own shocking defeat.
Without additional data, there is little point in debating the merits of this controversial theory. No doubt it will remain a matter of contention for years to come.