You *can* build a Guardian like a tank. Guardians get enough boons & heals to support themselves in battle and draw enemy aggression. You just have to experiment and discover the builds that work best for you. There are so many builds out there, so many viable builds... that's one of the major reasons why I love this game.
Well, different mobs have different aggro formulas. The way to draw ire with one dungeon won't necessarily work in another.
It's been determined that your highest chance to draw aggro is affected by having low hit points, low armor, rezzing someone who's down, proximity and applying conditions. That's not universally true, but generally so. So the mobs are actually smart enough to go after the squishiest folks. Rolling back, getting out of proximity and breaking LOS are good ways to drop aggro, though again, it's not 100%. They deliberately did not want combat to consist of "heavy armor guy stands here making fun of monster, healer dumps hp into heavy armor guy, DPSers kill monster," which I'm thrilled about.
I'm not sure why someone would WANT to simulating tanking in a game that specifically gives freedom from the same, but I've seen it done. I've got 2 very good low-hp guardians who do an amazing job of kite-tanking (which is really the only viable way to tank - you are NOT gonna stand there getting hit, you will die quickly and no one has big hp-dumping heals to stop it) and putting up reflective walls and boons and doing really amazing things. I'm not sure why they WANT to do it, but they do a good job.
IMO, it's best not to build a character for a traditional "role" (tanking/healing/cc/dps) but instead to build a character to be able to react to situations. Swapping your utility skills in and out depending on the kind of mobs you're fighting is super important. Just having a good grasp on your class and what it's capable of and how you feel comfortable playing will pretty much allow you to slot into a group and do a great job. I've taken level 10 characters into the Fractals of the Mists dungeon, which a pretty rough place - with GOOD players behind them - and we've come out winners without too many hiccups.
I've actually done an all-mesmer Ascalonian Catacombs run, which means we had class anyone would consider either a tank or a healer. It was pretty amazing and hilarious and full of pink lasers and butterflies.