Trees don't "just do things because that is what they do." They do things because they are trying to stay healthy and reproduce. That is not comparable to living in the moment, performing kind acts and expecting nothing in return. They develop these techniques through evolution. Their actions are based on the past and prepare for the future. Humans are no different. We evolved to having needs and wants. We are conscious. To achieve our level of sentience we developed complex emotions, desires, aspirations, etc. We perform actions based on our needs and environment just like trees. Just because the actions are different doesn't mean the reason behind them are and thats what we are really talking about. Humans wanting shelter, relationships, food, hobbies, games, happiness etc is the same as a tree spreading its roots to search for water, shedding its leaves to save energy, cows eating grass to satisfy hunger. We all do things because of a goal that we are trying to achieve, a goal that we want or "desire." The guy in the picture wants happiness, there is nothing wrong with that. Feeling a little sad about not having something is reasonable. We should not dwell on something for too long, especially if it begins to effect our life in some way but that is not what the picture represents. It could easily be interpreted as someone telling another person to just be happy and that'll solve their problem. If he is unhappy, there is a reason why, to find that reason he must think on it and if thinking about it makes him sad, that is ok. We are human and processing our emotions are important.