Make sure you are playing with people whose company you enjoy. Seriously, having a guild where you have real friends will get you involved in many aspects of the game you would not else care for. I can not stress enough how important this point is. Being in a very social and friendly environment means you will do things like (and enjoy doing them much more):
- Go play lower level zones just to accompany your friends, or vice versa.
- Check out Jumping puzzles/mini dungeons when your friends stumbles upon a new one, or vice versa.
- Do dungeons you've already went though just to play with your friends.
- Have the opportunity to try playing Dungeons in a coordinated and strategic effort instead of the randomness and amateur pugg style.
Set attainable goals for yourself
Setting goals for yourself that you can achieve on a regular basis is a very important part of maintaining interest in a game like this for longer durations. You have to realize that there is a lot of things to do within the game world that is not directly encouraged by the game itself. The game itself artificially encourages you towards some of these goals. For example completing the game, outfitting yourself in level 80 exotics, completing the map exploration and so on. But there are other goals that you can set for yourself when you have done these things. Examples:
- Working towards a Legendary (the most obvious one so I'm getting it out of the way)
- Getting a dungeon armor for the skin (if there's any you like)
- Completing specific achievements - may they be boss kills, mini dungeons or jumping puzzles. (these achievements in particular are good because there are many of them and they are relatively easily attainable, granting you a feeling of steady accomplishment.)
- If the Legendary grind is too much for you there are other cool looking weapons you can make in the mystic forge that are not as unattainable but yet require a little more effort then your normal weapons, I recommend you check these out.
- Maxing out crafting professions can be a fun little side show to the real game.
- Going after each of the world bosses is also a good and relatively easily attainable goal.
It's okay to divert from direction once you have it, but playing without it will result in boredom and the feeling of *There's nothing to aim for* which simply isn't true. The game just doesn't spoon feed you tons of objectives. Try to combine long term goals with short term goals and mix it up, don't overwork yourself on one thing or you will lose interest.
Mix up your type of game play
A LOT of people limit themselves to being only one kind of a player. Either you're a World VS World player, a PvE player or a sPvP player. This kind of thinking will guarantee in the long run that you will bore of the game. Mixing it up between all of them means it will take more time for you to divulge content. Which means it's more likely by the time you're starting to get bored ArenaNet will have had sufficient time to pump out some new content for you to enjoy. And who knows, maybe you find something new you like. And if you don't, no harm done.
Take a break from the game
Remember that Guild Wars 2 does not have a sub fee. There are other games out there and lots of real life activities to enjoy. As with anything, doing the same thing over and over again for hundreds of hours without variation is gonna bore you of it. It doesn't matter what it is, when you grind the same thing for thousands of hours it will get boring to the point where you want to rip your hair out. If you are a hardcore gamer I might suggest picking up one of the very good recent releases such as Hitman Absolution, Assassins Creed 3 or Far Cry 3.
So every now and then, play something different. Or do something different, go out with friends, watch a movie, go to a bar and get wasted. Don't obsess over the same game each and every night, you are doing both yourself and the game you want to enjoy a disservice.
I hope you can garner some wisdom from this wall of text and that it may help you enjoy the game more, I wish you good luck.