How do you know when you love someone?

So I've been wondering, how do you know when you're in love? Is it just something that you feel and you know you love them or is it more complex? For me personally, I have an older brother whom had a crush on a girl but after she had left the school district, he immediately had a crush on another girl the next school year. Are feelings supposed to fade that quickly or no? I feel like I have a crush on a someone but i'm not sure, I don't know how you're supposed to know when you are in love with someone, sure you feel it but how do you know for sure? I haven't experienced being in love before so I wouldn't know how I feel for sure. It could just be that he's nice, takes care of animals, does art (like me) and makes me feel like i'm not being annoying by disturbing him but that could just be things that I admire about him as a friend, especially since the only other male figures currently in my life (my brother and dad) make me feel like I am annoying and rarely act nice to me. I don't know, can you guys help me out? What does it mean to truly love/be in love with someone?