Well it's a show that just has it all, absolutely hilarious and at the same time it still has the ability to make you so damn sad, it's amazing... Remember when they lost 3 patients to rabies and Cox's guy goes last, where he throws a fit? Damn near cried during that ending :/ (If I remember correctly it was S05E14?)
Exactly. It can be very funny, full of light humor and fun, then BAM, it's emotional, sad, and brings out other emotions. A lot of this happens in mid to last season, though. The first few seasons are more comedy focused on comedy.
For example, the episode you're talking about is called "My Lunch", and takes place during season 5, as you said. Like season 4-8 are the best, IMO.
My favorite episode without even a shread of doubt is season 8 episode 2 called "My Last Words". It's funny, but is more sad and brings out more emotions than all the others.