Wow! I've been to China, Vietnam and Japan. No future plans yet.
Which country that you feel is the most challenging to travel alone?

I think they are offended because you "should" travel with your boyfriend, not alone if you're in a relationship. That's one of the bullshit rules of society. I totally disagree with this rule.

Regardless of you're in a relationship or not, if you want to travel alone because of whatever reasons, you're free to do so. It's best when your boyfriend understands you.