A word or idiom to describe someone who has fears or reservations when others are doing or thinking the same

Question

I vaguely remember a quote floating on the WWW that says

Be careful if everyone else is doing the same.

Exaggerated example:

Tom loves to ride. He has an impeccable passion in riding and motorcycles. However, if he encounters someone or a group who shares the same level of passion as him, he feels inhibited. He reacts as if he isn't as interested for fear if something will go wrong if too many of the likeminded gets together.

How would you describe people like Tom? Is not the same as unconventional, non-conformist just in case you think along that line.

Much Thanks

Answer 1

Whenever you find yourself on the side of the majority, it is time to reform (or pause and reflect).

Mark Twain, From a 1904 entry in his notebook.

However, and it's a big however, this quote is almost always used in a manner that would have Twain rolling over in his grave. Here is Twain's concept of majority and minority from his unsent letter to Bayard Taylor, June 10, 1878 as published in Mark Twain at Large by Arthur L. Scott.

When you set aside mere names & come down to realities, you find that we are ruled by a King just as other absolute monarchies are. His name is The Majority. He is mighty in bulk & strength ... He rules by the right of possessing less money & less brains & more ignorance than the other competitor for the throne, The Minority. Ours is an Absolute Monarchy.

Answer 2

I think the best word I could come up with is self-conscious

excessively aware of being observed by others.

I suppose insecure, anxious could do too.

As far as idioms, I would like to refer to this answer.