Why sperm be come liquid and how get treatment ?

The semen is made up of sperm and seminal plasma fluid. It is thick right after ejaculation so that the semen stays in the vagina for longer and does not leak out. After several minutes it becomes watery so that it allows the sperm to move to the uterus and the Fallopian tube easily.

You may find the following link to be helpful as it discusses everything one to know about sperm.