From my experience, a lot of the time it comes from having a crush on someone and not having the balls to tell them. There are some days where you just want to tell them, just so that they know how you feel, or because you might have a feeling they might like you back. But then there are other days where you wouldn't even think about telling the person because of your self-confidence. It makes you think: why would they even like you? There's not many good qualities about you that would get you noticed by that person in that sort of way. You can still be good friends with that person, but still have the feeling that you would have no chance with them. I'm not sure if others feel this way, but sometimes I feel like the person I have a crush on wouldn't go for me because I'm not smart enough, that my grades aren't good enough. Maybe they're doing some science course and you're doing a media course or some other course that you feel doesn't require as much knowledge.
Or maybe you feel like they wouldn't go for you because your life doesn't have as much structure as theirs does. Maybe you have no idea what you want to do with your life, even though you may be studying and they seem to have theirs on track and know what they want to do. Maybe you're a bit like me, living at home still, not having very many friends, you're quite shy so it's hard to make friends, the friends you thought you had aren't actually there for you when you need it, so now you feel inadequate about yourself. Which links back to the self-confidence issues.
It's a crap feeling having feelings for someone who may not have them back. If you're like me, then you probably think about them constantly, what they may be doing during a certain day. Or if they're upset, you want to be there for them, comforting them and making them feel better. Or sometimes you might go places with friends and you might wish they were there with you. Or if you go on family holidays and you wish so much that they could be coming with you, making them feel like they're a part of the family.
It makes you think of all different scenarios you wish could happen. Taking them on a fancy dinner date, taking them to see a movie they really want to see, or similarly, watching their favourite movie with them. Going to the beach and enjoying the warmth and water with them. Staying in nice hotels with them, but also enjoying going camping. Going on bush walks with them or being a good partner and putting up with going shopping (for all the guys out there).
Now, the only advice I have is advice that was once given to me. If you really like someone, pluck up the courage and tell them. Even if they don't like you back, you might look back and regret that you didn't tell them. If you're not up for that piece of advice, then be sure to hang out with them as much as you can. Perhaps take them someone you don't take many people and tell them that. Make sure they know how much that particular place might mean to you. Enjoy their company and if possible, try to tease them every now and then. If you feel comfortable around them at all times, embrace that feeling. Just take your time and it might pay off one day!