1)they will never love you back. Sad but oddly, easy to recover when you meet someone else. Usually resulting in you thinking "screw you your loss"
2)Society won't let you be together. This hasn't happened since Romeo and Juliette, so I'm not gonna explain it.
3)You love eachother but distance separates you. The worst one. The one I've personnally experianced myself. I lived on California coast, he lived in Reno nevada. We met at camp, loved eachother completly, but then camp was over.You turn into a mess for about 3weeks to a month. Then you think about them, but no more tears. You never fully recover, you just have to move on. Any love will destroy you, so just avoid it if possible.
2)But...we can never be together, everyone forbids it
3)I miss you.
"Damn straight. He's got a girlfriend who he's crazy about! It's a forbidden love."