As we know, Tauriel is a bit... uh, let's say fickle. We have a scene of her mooning about over Legolas, followed nearly instantly by her falling for Kili. That's one of her personality traits. The other? A violent hatred of orcs, to the point where she'll disobey direct orders so she can kill some more.
Oh, wait, one more: she doesn't think locking the gates and letting the world go hang is a good idea.
Thranduil, now... is a raging xenophobe. He kind of was one in the book, but he's dialled it up to 27 here. He's actually nicer towards Thorin, but refuses to even consider fighting outside his borders - and is racist towards Silvan elves. Which is weird, what with his kingdom being made entirely of them.
So where'd that come from? Well, let's take a look at Thranduil's history:
-Born and/or lived a long time in Doriath. Doriath was populated almost exclusively by Sindarin elves. It was defended by a magical barrier which kept evil entirely out, but left the rest of Beleriand to get utterly wrecked. Sound familiar? Movie!Thranduil's strategy is Thingol and Melian's strategy.
-Driven out of Doriath by the combined violence of dwarves (who killed Thingol) and Noldor (who killed everyone else). Gosh, I wonder why he hates dwarves so much? The death of Thingol, by the way, is the origin of the elf-dwarf feud. And a very shiny rock was involved...
-Moved with his father to Mirkwood Greenwood, where they spent a couple of thousand years ruling a bunch of Silvan elves before they...
-Went to the War of the Last Alliance, where Thranduil's father was killed. That war, by the way, is the one that was caused by the Noldor and the Numenoreans feuding with a renegade Maia who the Valar failed to properly deal with. Literally everyone is to blame for Thranduil's father getting killed.
-So now he's stuck in a woodland realm which is slowly going to pot, in rather grotty halls (compared to the grandeur of Menegroth in Doriath - or of Erebor, for that matter; this is why he wants Thorin's gold and gems, to make his home into something he can think of as worth having. It may also be why he drinks so much).
-He's also surrounded by his social inferiors. Now, Thingol was quite good at dealing with Silvan folk - some of them saved him in the First Battle of Beleriand, after all - but they were the ones who kept heading west, even if they took a while to do it. The elves of Mirkwood (and Lorien, for that matter) are the ones who gave up completely. Compared to the Sindar, they're layabouts. And the Sindar are the only elves in Middle-earth who never compromised. The Noldor got kill-happy in Aman. The Silvan elves, and the Avari if any of them ever show up, wussed out of the Great March. The Sindar? They're the ones who stayed in Middle-earth to search for their king - and found both him and his Maia bride. Even if he wasn't born then, that's a decent amount of cultural pride.
Does Thranduil have a reason to be racist against Silvan elves? No - but given that he's busy hating literally everyone else, I rather suspect he's narrowed his 'acceptable people' definition down to 'Sindar'. He does, at least, seem not to be too angry at Elrond (at least, he was willing to send his son off to visit a few decades later), and Elrond is the last non-mortal descendant of Thingol and Melian.
But what about Tauriel? We know why she hates orcs so much - her parents were killed by them. We don't need a reason for her to think ignoring the world is a bad idea - it is a bad idea, and anyone other than the brainwashed-by-his-racist-dad prince would agree (and, actually, Legolas does agree - or at least go along with it). But the bizarre romance angle?
Well, let's look at her role model. For 600 years (possibly her entire life, but a long time at any rate), Tauriel has been raised by Thranduil... who is racist against her and has no qualms about saying so. Her emotional contact with the person who shapes her worldview consists of him saying 'You are worthless'.
In fact, if she was only an infant when her parents died, Thranduil may well have given her her name. So, when he adopted a poor little (Silvan) orphan, what did he call her? ‘Forest girl’. Even if you ignore the phonetic resemblance of ‘Taur-‘ to ‘Sauron’ (yes, you bet it occurred to Thranduil), ‘Taur’ is a word for forest used almost exclusively in the negative sense. ‘Taur-nu-Fuin’, Mirkwood, or ‘Taur-e-Ndaedelos’, Forest of Great Fear. The more positive ‘Eryn’, as in ‘Eryn Galen’, Greenwood, would have been kinder – had Thranduil felt a scrap of affection towards his charge.
So I figure, she's a bit messed up. She doesn't know what affection feels like. She has four emotional responses in her repertoire:
-Hatred. She hates orcs. Hate hate hate. Of course she does – they killed her parents.
-Nothing. She's a captain of the guard - but we never hear anything about the people she serves with, or commands. They seem to be little more than tools in her quest to wipe the orcs off the face of the planet. Look at how she treats... is it Bofur she grabs the athelas from? He's a tool, useful for her goal, then ignored.
-Sorrow. She is sad at... well, Thranduil. Probably she doesn't like people being racist about her when she can't hate them.
-????. There's a fourth emotion she hasn't felt very often. She feels it about Legolas (her pseudo-brother), and about Kili (because he's young and she first met him by saving his life - he's almost like a younger brother, really). Most people would just call this 'affection', or even just 'liking someone'. Tauriel? She's got zero emotional baseline. She calls it love.