The sincerity we always give us will not always get what we expect, because everyone's hearts are different. Nothing is the same, we can be sincere because our hearts feel, but when we want to know the feelings of a person we will never know whether his sincerity or just a lie.
Here the sincerity is not derived from family, lovers, or people we care about. He is someone else, whom we may only think of as friends, and will not be more than a friend, but he has an extraordinary sincerity for us.
In life there will definitely be people who are genuinely genuine towards us.