Please go read the string Why am I tired and depressed?
It addresses the issues of what impact quitting can have on a person feeling depressed, but also explores how it is no longer prudent to just assume that all sad feelings now are just reactions from quitting smoking. While it is possible that quitting is the cause, it is equally possible that other factors in a person's life can be responsible for such feelings. Read the string Why am I tired and depressed?
and if you have follow-up questions feel free to get back in touch.