Emotional Maturity is Physical Maturity

Lots of speculative fiction includes non-human characters whose natural (or supernatural) lifespans are way beyond that of a human. This is a cool concept, and an area ripe for culture clash when humans show up. These long-lived creatures also seem to bear a permanent (or at least long-running) physical resemblance to teenage or young adult humans. Again, this could be an intriguing feature of their species, and definitely makes sense in a number of folklore and fantasy contexts.

What doesn't make sense, however, is that these characters seem to have a natural affinity for teenage behaviour. They act like angsty adolescents. They hang out with-- and often romance-- teenage characters.

This, in and of itself, could be salvageable. Maybe this alien species lives for 5,000 years, and between the ages of 600 and 900 go through a developmental period analogous to human adolescence. In that case, it makes perfect sense that these individuals would prefer humans who were at a similar developmental level.

On the other hand, there are species for which such a timeline doesn't make sense, unless you work in some additional explanation. Most prominently, a 19-year-old who gets turned into a vampire and wanders about for 150 years should not be stuck in a 19-year-old mindset unless this is a feature of vampires in your world. Even if they don't physically age, they'll accumulate a lot of life experience.

This brings me to my final point. Even if you have a long-lived species which experiences it's own equivalent of being a teenager, these characters will still have lived a much longer time than their human counterparts, and thus will have a very different perspective on things like the passage of time, how significant events are to the 'big picture', how many experiences they've had, etc. This should really cause a lot more issues as far as their relationships (of all kinds) with humans.