Job 3

So this passage confused me the first time I read through it and I'm not sure I fully understand it just yet. In the preceding passage, Job is stricken with leprosy. His wife rebukes him, his friends confront him, yet he still will not give in to their temptation of just simply giving up... At this point, Job falls on his knees, cuts himself with a broken clay pot, and begins vehemently cursing the day of his birth.. What reason is there in cursing his existence? It is interesting to note that though he curses his birthday, he does not curse his God. It still is sinful what Job did because it was vain and he is later reproved by God for it, but I still am unsure as to why, of all things, Job chooses his birthday to swear off..