Okay, I realize I may have lost some people already. This should clear things up.
So basically your lobster is your soulmate. I'm a firm believer that everyone has one, it's just a matter of finding them.
1. Ross and Rachel (Duh)
So if you know anything about friends, you know that the Ross and Rachel story is pretty much a central theme throughout the entirety of the series. Ross has had a crush on Rachel ever since she and Monica were friends in high school. The extent of his feelings for her is made known in the episode The One with the Prom Video, and this is where we see them officially get together.
They date for a little while until about halfway through season 3 when Rachel suggests they "take a break" and well, you know the rest. They spend the rest of the series in an on-again-off-again sort of relationship. They both date other people as well as get back together for brief periods, like to drunkenly get married in Vegas,
and to have a baby together,
before they FINALLY get back together for good in the iconic series finale, where Rachel gets off the plane.
Although Ross and Rachel's relationship was never perfect, and it took them TEN seasons to realize that they were really meant to be, they found each other in the end, and that's what having a lobster is all about.
2. Chandler and Monica
Chandler and Monica were a couple that no one saw coming but made complete sense once they got together. They had both been in serious, long-term relationships before discovering that their lobster was right under their nose. What started out as a drunken hookup in London during the weekend of Ross' (second) wedding bloomed into a beautiful relationship that I would appropriately describe as #goals.
Although Chandler and Monica had known each other for years (Chandler was Ross' roommate in college), they didn't get together until the end of season 4. While you might be thinking that seems like a questionable amount of time for two people to realize that their soulmates, I think that part of the reason that Chandler and Monica worked so well together is because they had such a solid foundation together as friends.
3. Phoebe and Mike
Phoebe is the only friend we see to look outside the group for true love. She meets Mike on a blind date that Joey sets her up on, and the rest is history. Although they don't meet until season 9, they instantly click with each other; Mike turns out to be just as quirky and odd as Phoebe, just maybe in a more muted sort of way.
Phoebe and Mike date for about the course of a year until they get married in the last season and have the most beautiful, spontaneous wedding where they've written their own vows, which pretty much sum up their relationship perfectly.