Why is joining online gaming sessions flawed?

Seriously, why do I get so many errors when I try to join someone's gaming session or vice versa on literally every game? How come developers can't just make it work flawlessly? I'm wondering why. Is it incompetence or actually difficult? What causes it? It seems to happen on every console as well. When players are online why does it still fail to join each other so often?

It's mainly the conection of the 2 consoles. Sometimes it's the host's connection. In which case on my PS3 I use a lag switch just long enough that someone else can become host. As you pointed out it's a big pain and in some cases it's the server itself. Take Black Ops Wii for example the game was once filled with well over 15,000 players but over the course of 4 years and less and less players getting on it got weaker to much data and people being on at one time. I noticed it when there were just 3000 on yesterday. Half of my lobby couldn't hear what I was saying. It was mainly only about ReVx but still. Which reminds me since I know that moron will see this. ReVx is still alive and well and has 37 members on the Wii incounting. Were not just the Wii were PSN and XBL as well. If you honestly thought by hopping a account that I use as a Anti Hop and clearing the allys of said profle that I use as again a fake account for my Anti Hop was gonna syop the growth of the clan your a bigger idiot then I thought.

Yeah, the same thing happened to me and Cant Stop Anons last night on MWR. He had better ping than I did. He couldn't join me, but I could join him.