A lot of it is being observant - and doing research. Many businesses are based on solving problems the founder had him/herself (e.g. "this seems busted - there has to be a better way") or things they recognize because they are in that line of work (e.g. "man, this could be some much easier if thus and so happened"). That said, many other very successful businesses start from identifying a huge market area and looking for problems worth solving - and seeing if they can be, profitably.

Then you put a smart team against it and iterate.

There's no one right answer.

That said, simply using self reference criteria isn't often the right way to go. I spend a lot of time talking with founders or founding teams who are recent grads trying to do things for that market because they are most familiar with it, even though it might not be the best or most profitable fit, for instance.