I'll preface this by saying that I am Canadian, not American, and I do always vote. I will find a way to make a choice and vote in our next election, but lately have been understanding why someone might not:
Everyone who has even a remote chance of winning has at least one position that I find entirely unacceptable. Like, I cannot in good faith vote for this person because this issue is an absolute deal-breaker for me. If I'm throwing my vote away by writing someone in, why even leave the house?
I moved from the UK in my early 20s, prior to that I was young and stupid, so I neglected to vote there. Then I moved to America and started the green card process, and didn't feel it was right to vote for things back in the UK as it wasn't my home anymore and it wasn't my place to say what should happen there. I finally naturalized around a decade after I moved here, and immediately signed up to vote. I actually cried at the polling station because I was so happy to vote for the first time ever!