"Fear makes the wolf bigger than he is."
---- German Proverb
And the cliche, but what I believe to be very apt to the question,
"The only thing we have to fear is fear it'self - nameless, unreasoning, unjustified, terror which paralyzes needed efforts to convert retreat into advance."
---- FDR - First Inaugural Address, March 4, 1933
In the beginning, that is exactly what I, along with all the other answerists, believed. It was the characters' fear giving it the strength when they were frightened, such as the moments when the characters were killed due to fear or being mentally weak. When the fear of the character was absent and they stood or fought back, the "monster" did not have the strength or ability to defeat or overcome the LOSTee.
"The oldest and strongest emotion of mankind is fear."
--- H.P. Lovecraft
The popular theory also made some sense of the Great Others War.
"Collective fear stimulates herd instinct, and tends to produce ferocity toward those who are not regarded as members of the herd." ---Bertrund Russell.
It made the only plausible sense of experimentation and murder of the New Kids on the Block LOSTees by the Others. Plus, the LOSTees did not, and apparently after numerous flashbacks and revelations, still do not even know each other.
There are many reasons to be afraid everyday on the "island." Afraid of themselves, each other, each "other", and having to survive.
"Only when we are no longer afraid are we free to live."
The monster seems to appear randomly. At the beginning it was a great mystery. Then, after creating an"other" group of people, it seemed the monster lost its purpose or its neccessity. But, I think, it may be that the monster only appeared when in the moments of the greateset uncertainty or vulnerability; thus the moments when were most afraid.
I believe the monster has to be "fear".
In retrospect, I do have to say that after almost every twist or question has been explained to us scientifically; it seems like a desperate move to toss the monster up to voodoo doll magic.
Plane crash was magnets, the others are mid 30-40 retirement community gone awry, there is a second island everyone missed, and countless (plane, balloon, boat) crashes on an unknown island.
The real monster answer better be impressive.
Or maybe the show is based on the whole "we are living in a sleeping childs dream, and when he wakes we will all be gone" thing.