Dear, just observe this guy. If he is good, tell him directly what you feel for him. Don't be ashamed.It's better to try than do nothing.Anyway there's no harm in trying..it is better to be frank than keep something in your mind without expressing your true feelings. If he likes you then go on ..if not leave it. Accept the truth that he's not meant for you. God prepared Mr Right for you out there. Be positive always.
Why do you say, "Shouldn't"? Is he married or engaged? Feelings are something that we cannot control. If you already know that you are not supposed to think about this guy for whatever reason, then you must indeed be going through hell…
If at all possible, you could ask one of your close friends or relatives to have a talk with the guy, if he is free and available, and ask them to convey your feelings for him. If he reciprocates, then you would have attained your heart's desire.
If he doesn't, for whatever reason, then what you can do to stop thinking about him is to start a completely new activity, something that you have not tried before. It can be sewing, or skiing, or ice skating or even Para sailing. When you do this, your mind will be engaged in trying to learn the new skill, and you will gradually start to forget about this unattainable guy.
Love him anyway! That's what I'd do! Love is great!