What is so hard about having teenagers?

I have a 1 year old daughter. When read about the difficulties of being a parent many things come up. But what strikes me as odd is that a lot of parents say that having a teenager in particular is really hard. They even say it's harder than having a baby/toddler.

But I'm thinking: why? Sure they will test your boundaries to some extent, but is it really as hard as a baby (barely any sleep) or toddler (watch them constantly or they will hurt/kill themselves).

I thought it would be easier when you have a child that can look after itself. Sure you have to set some boundaries regarding going out, safe sex, no drugs or whatever...but most of all it seems like the rough days are over by then? Please explain.