Do you like Adele? This statement is confusing probably because of the way she constructed the sentence. More grammatically correct: sometimes love lasts. Sometimes love doesn't last and that hurts. To say that something lasts, means it never goes away or wears out. And the only thing I can say about why she wrote it this way is just that she is an artist, and that is the best way she could tell you what she was feeling.

What is there not to understand? It means that sometimes love lasts for a long time, I guess in this case presumably for life, but that sometimes a relationship goes bad and people get hurt because of it. Nothing last forever though...it only lasts for a lifetime, unless you believe in an afterlife.