The Second Noble Truth is that all suffering is caused by craving. When we look at psychological suffering, it is easy to see how it is caused by craving. When we want something but are unable to get it, we feel frustrated. When we expect someone to live up to our expectation and they do not, we feel let down and disappointed. When we want others to like us and they don't, we feel hurt. Even when we want something and are able to get it, this does not often lead to happiness either because it is not long before we feel bored with that thing, lose interest in it and commence to want something else.
Put simply, the Second Noble Truth says that getting what you want does not guarantee happiness. Rather than constantly struggling to get what you want, try to modify your wanting. Wanting deprives us of contentment and happiness.