Why leaving is difficult!!?

I am just not able to understand,why leaving Pune for this summer project was more difficult than the times when I Left Pune for vacations during my Medical course. Especially when a friend of mine replied to my goodbye sms saying that,I'll be missed.Really knowing that you could be missed by someone,makes leaving those people more difficult.Why is it so that these bonds of 7-8 months appear to be so strong? Really you never know,where you would find so good and close friends!!!!