Why do we notice things more when it's happening to us? Human psychology?

why is it that, for example, when you fall in love all the love songs seem to come on the radio and you notice everything that is about love? Is there a rule or psychological reason as to why this occurs? Another example is when you have a certain problem, maybe it's about your hair or social life, you seem to notice that other people have the same problems, problems you didn't see before. Why is that?