Why people cry?

Some years back i had a big question pounding on my face - why people cry ? I used to see my mother crying for many unwanted  occasions in my life which were actually nightmares for me. I did not realize the exact meaning of sorrows one gets when something unwanted happens in his or her life. 

Those earthly feeling comes when i fell into love in my engineering year. I sometimes thank to God to give me the courage to say it -I love you to somebody. I don't really know what would happen if i did not try my best to love somebody. I would be a pig , i think, because before my love era started i was a ruthless king of own domain, i don't know why people  lough and cry. i just did one thing properly - to get ready for any exam and  and do well in that . I  had no idea what the  world without-me means?

I do understand the meaning of emotions when i feel those ones .And that makes me a complete man ..