The true magic of Friends is that the jokes never become stale. No matter how many times you watch the episodes, you find yourself laughing out loud at the same moments and often you'll pick up on something you missed first time around. For example, Paul the Wine Guy is also Paul the Whine Guy, gettit?
Few shows, if any, from the last 20 years have the same rewatchability factor as Friends does, packed as it is with layered gags, hidden references, and of course the odd mistake along the way (no series is perfect, unfortunately). Re-watching the programme for the 50th time on Comedy Central, Netflix, DVD or whatever, you'll often pick up on something you didn't before, or laugh at a part you never really understood when you were a bit younger.
From sight gags to continuity errors, there's plenty of stuff in Friends you probably haven't noticed, no matter how many times you watch it. It might just be the odd thing in the background that you didn't spot, or a joke that went over your head.
Like Joey in a group conversation, it's probably best to just nod and laugh along. Unlike Joey, you don't need to spend $50 of someone else's money in order to understand, and it isn't just limited to things beginning with the letter 'V'.