Is it necessary for couples have something in common? There are two schools of thought on this:
A. "But we have so much in common!"
This is a nice idea. A couple that has tonnes of things in common, share the same likes, dislikes, hobbies, etc. They don't even argue that often. The problem with this relationship is that it can get a rather boring and couples will tend to lose interest in each other.
B. "Opposites Attract, and then kill each other..."
This one is pretty much self-explanatary. A vegetarian catlover dating a meatlover who's allergic to cats. Little or nothing in common beyond the fact that they're both homo sapiens. Arguments are bound to set in, but at least they're interesting and sparks are flying.
Lets say for example that you like going to the horse races and betting on the ponies. This isn't for everyone obviously (especially people allergic to horses) and the percentage of people out there with a gambling problem (or the potential for one), or simply a strong moral belief in not gambling, is going to put a damper on bringing your love interest with you on a trip to the racetrack.
For myself, I've discovered that the horse races is a good place to meet women... but they won't necessarily be the type of women you'd want to marry. They will likely fall into the B category above and the relationship either won't last very long or is at least doomed from the start. Yes, you may have racetrack betting in common... but the rest of your interests will likely clash.
I suppose a gambling habit would be one way to break up with a girlfriend. Nobody wants to date a loser who pisses away all their cash gambling on the horse races.
Certain hobbies vary in popularity: Dungeons and Dragons, Video Games, computer games like Counter-Strike or Online Poker... not everyone plays these games of course, but those that do are likely addicts. Some games are just very addictive, but trying to introduce a friend to a new game can be a tricky prospect. Depending on the person they simply may not be interested.
For example I introduced my girlfriend a year ago to Dungeons & Dragons Online (I have to warn you it is very addictive), and she and I have played it regularly ever since. However... when I tried her favourite game, The Sims 2, its not the same result. I just don't find the game that interesting. The part I enjoy the most is building and designing the houses... actually controlling the people is boring as hell to me.
And you know what you do when you're controlling the Sims in the game? You're telling them when to eat, sleep, shit and go to work... and you work on their relationships. You're basically playing matchmaker with little computer generated people. What a waste of time. I'd rather spend my valuable time with my real girlfriend.
My advice? Having things in common are nice, but its not the be all and end all of a relationship. Communication, sex and faithfulness are much more important.