Wondering

Everyone is bond to find someone they have once loved or still possibly love? How do we know when it's true love and know that it will last? I don't think it's ever possible to know when that can actually happen. Teenagers and adults become devoted into relationships either for fun or because they see themselves with that person for a lifetime. I've been into relationships where I felt that I can possibly see a bigger future with them. Sometimes it would last for a very long time but other times, it's just another failure. People throw around the word "I love you" and never really know when it's the right moment to tell someone they love them. Sometimes people loose feelings for the person they've once said they loved and breaks his/her heart. Is it that they have never loved them or didn't it really just die out?