Actually to be honest, all the 답변 before me are part of love, but it is also part of attraction to the person. I mean one does get 나비 and feels nervous around a crush, but that does not mean 당신 would feel the same way when 당신 are in 사랑 with someone and they are in 사랑 with 당신 back.
Truth be told, the feeling of being in 사랑 is different for everyone. The same common feelings like wanting to be with the person always and all that 나비 are still there, but when 당신 are in love, it all intensifies, beyond what 당신 imagined could ever happen before.
But above all else, being in 사랑 means putting someone before yourself and also telling yourself that I am worthy of such 사랑 from this person and I as a person would be able to give that 사랑 back. It's all about feeling that ability to give and receive as well as to care, sacrifice and respect.
당신 know you're in 사랑 when 당신 think about that person all of the time, when you're 심장 starts beating faster every-time 당신 see him/her. 당신 want them so badly it makes 당신 want to scream. That's how 당신 know.
You're in love- and I mean it- when 당신 can imagine yourself spending your days at the side of your loved one. When 당신 don't mind making sacrifices, when 당신 know, 당신 know, cause your 심장 tells 당신 and your mind agrees.
