Think before we talk

Yesterday, my friend told me the encounter that made him depressed for one whole day.
As you know nowadays many Internet users like to join different type of social network. My friend John had joined one of the social network and added many on line friends. What made him so angry just because of the following comment:
" John, you look very old as if you look likes 40 years old, I do not want to be your friend." seeing this, my friend felt hurt and depressed. He looked like totally lost the morale.
I consoled him: " John, please take it easy. This is the reality that in the real or Internet world, some will like you, some will not. Do not keep it to the heart, you just ignore it. It does not matter, surely some one will accept your invite. Please smile, you still have a lot of interesting things to do....."
After that, he really smiled and said:" Thank you very much. "
My friends, do you know why John felt depressed while seeing the comment?
My friends, I like to write more but I need to stop here because I feel hungry now.
One point I like to emphasize : Think before we talk or write, especially we must not hurt other's feeling unnecessary, am I right? Let's smile and empty all the worries.