That common question is something that as I watched Clannad After Story constantly came to mind. And that makes sense, since the main character (Tomoya) had those same thoughts. To Tomoya, everything bad that could happen in his relationship with Nagisa and his life in general does go wrong. There are moments of happiness that he experiences, but something bad is soon to follow. After awhile this kind of reached it's limit to me, honestly how much could one guy endure? Simple answer is a lot.
I don't want to go too far into detail about what happens in the story as I feel that would spoil everything and take the impact that moments in the show will have on a person. Not everything in After Story is sad, there a lot of happy and funny moments to keep you from getting too down about where the story is going. That being said do be prepared for a few tear jerking moments, unless you have no heart.
The one problem, and it is a small problem, is how the story ends. In a typical fashion, it does end on a happy note. The way it does this kind of makes everything that happens in the series a little bit of a mute point. However, you do get answers that you may have been wondering about even from the beginning of Clannad. It wraps up nicely, and even though the main character still has memories of everything that happens it just leaves you with a nagging feeling that a lot of what you've watched was for not.
That small problem aside, After Story continues the story of the great characters from the first Clannad and if you liked them in the first series you will want to see how things unfold in After Story. The animation is top notch as is expected from a Kyoto Animation series. I can't comment on the English Dub that Sentai has done as I watched it with the original Japanese language on.
For anyone who has watched the first Clannad and enjoyed it, recommending After Story is simple. If you haven't seen Clannad but are interested in Romance/Drama series, After Story may be a little rough to get into as the set up of a lot of the characters isn't there, although the series does get the viewer introduced and explained about things that had happened in Clannad in the early episodes and is worth it if you can get into it. For those that don't enjoy series that have characters that is slow paced and has a emotional story you may want to stay away.