If Data cannot be happy or sad, why does he try to be more human and is wanting an emotion?
Because Data's purpose was to be more than the sum of his programming
The Memory Alpha page on Data contains a few useful pieces of information:
"I chose to believe that I was a person, that I had the potential to become more than a collection of circuits and sub-processors." – Data, 2369 ("Rightful Heir")
Data asserted that he did not only perceive data and facts, but also the "substance" and "flavor" and other ineffable qualities of the experience, which would be lost when downloaded to a conventional computer. (TNG: "The Measure Of A Man")
The context of that first quote though is much more revealing:
DATA: Yes. The Starfleet officers who first activated me on Omicron Theta told me I was an android, nothing more than a sophisticated machine with human form. However I realised that if I were simply a machine, I could never be anything else. I could never grow beyond my programming. I found that difficult to accept, so I chose to believe that I was a person, that I had the potential to be more than a collection of circuits and subprocessors. It is a belief which I still hold.
Hence, his desire to grow beyond his programming is what makes Data strive towards becoming more human.
Emotions are only one facet of humans; Data acknowledges that he was not merely a machine, but a being with capacities far exceeding a normal machine. Many of his qualities, especially his appearance, were already human. He wanted to go beyond the superficial, though, and become more human. Note how Data has friends, notably Geordi; a very human quality. Data doesn't need emotions necessarily to form a friendship, so in this aspect he can become more human without the emotion chip.