Lust

Well, it sort of depends. If you're married or in a relationship, lusting after someone who is not your partner, can be hurtful and dishonest. Everyone feels attraction to other people at times, it is only human, but if it is getting in the way or taking the place of your relationship, then you need to reevaluate why you're doing this with that person other than with your partner.

If it's unattainable and/or inappropriate, ex: celebrity, married or otherwise taken person, that's also not the best thing.

If you're single, well, as long as it's healthy and not intrusive, negative or obsessive,then why not let that person know you're interested!

If you're a minor, while it's perfectly normal to have these feelings, just make sure they're appropriate.

what is bad when you act on it 

Since your profile is closed, I'm presuming you're under age.

And in that case, what you describe as "lust" is perfectly normal. It's hormones. At this stage of your life, your hormones fluctuate from day to day (sometimes even more frequently) and that's what causes you to have "lustful thinking."

I would be concerned about low self esteem if you acted on your thoughts and began randomly sleeping with people or making out with them no matter what your relationship status - but if you're not doing any such thing, then what you have are harmless fantasies and you can safely indulge.

Eventually, you'll meet someone with whom you want to experience your "lustful thinking" in person. When that finally does happen, please make sure you use protection so as to prevent unwanted pregnancy or STD's.

Until then, enjoy your thoughts.

I couldn't possibly improve on JKgrandma's answer. She's right, it's all normal. Hell, I'm 59 and I still get lustful thoughts all the time. If you're a teenager, they can be quite strong.

It's being human.