Everyday attachment
You go to the office on a particular day and you have a great time, all your jokes are timed perfectly, and people are generally enjoying your company, you feel great about life, and you expect every day to unfurl like that but the next day is very different, people are busy and nobody has time to pay attention to what you have to say and so this now becomes the cause for sadness. One of the biggest reasons for sadness in people's life is this everyday expectation about life to unfurl in a way to suit their requirements, but life has a mind of its own, it does not believe in repeating, algorithm of life is very dynamic, every moment is different, instead of enjoying the freshness and the ride we spend all our lives attaching concepts to it and as a result don't enjoy the rollercoaster.
To make life more enjoyable we have to learn not to attach ourselves to a particular experience and get sad if things don't work out that way, this is easier said than done however since we are brought up in a society that has concepts attached to everything, everyone believes there should be purpose attached to everything you do, people who are looking for the purpose of life are trying to solve a psychological problem with philosophical solution, it hardly ever works.
To get a new operating system for the mind that takes joy in every moment without any preconceived notions one has to sit down and meditate, meditation like religion is a medicine and not a diet, once one has made significant headway the difference between formal hours of practice and the rest of the life is fully mitigated, but until such time one has to meditate and you may start with 5 minutes at a time.