There are a million things that go into each one, consciously and subconsciously.. Some have been more on a whim, others have been mulled over for years, some have been a combination of both, if that makes sense.
Here's an example: I'd wanted a Poe tattoo since I was probably 13, thought about it forever and had the chance to get tattooed by Todd Noble last year. So I made an appointment, he drew something up and we put it on. Encompasses everything about my goals, inspirations, fears, interests...and then got it done in what seemed like a split second. Weird.