Tom's problem was that he lacked confidence; Ed failed for precisely the opposite reason
important
special
chief, main, major, primary, principal
only, simple, sole
The only reason I didn't become a professional golfer was because of my family commitments. I was never good at playing the trumpet for the simple reason that I never practised
real, underlying
She did not tell him the real reason for her change of heart. The underlying reasons for these differences will be explored in depth in the next chapter
very
There is an assumption that a state will protect its citizens. That is the very reason for the existence of states
apparent, particular
He was attacked for no apparent reason
no earthly, (no) possible
Surely there is no earthly reason why you wouldn't want to come with us?
clear, obvious
The reasons for her decision soon became clear
unclear
logical
understandable
justifiable
inexplicable, unexplained, unknown
For some unexplained reason the pilot jettisoned all his fuel shortly after take-off