Klena
Elena grew up with the Mikelson's and she was close to them all. But she was closest with Klaus.
Whenever her mom would send her out to go get a bucket of water, Klaus would spot her and walk her down to the water hole. Or when she would take a stroll in the forest Klaus would always ask to accompany her, he claimed that it was to protect her.
So that was how Elena saw it, but over time Elena found herself falling in love with him. His small acts of kindness touching her heart in ways she didn't think was possible.
Her twin sister Taita would always tease her about Niklaus and how one day he was going to ask Elena to court him. Elena of course would brush it off and try to ignore her sister but she always wished for that day to come.
Elena was no stranger to how Mikeal treated Klaus, no one was. But that is what made Klaus fall in love with her, the fact that she was always there for him. In fact Klaus remembers the exact moment he fell in love with Elena; Mikeal had kicked him out into the cold, forced to sleep outside that night, Elena saw this and she brought out all of her blankets and slept out in the cold with him. Klaus felt guilty, he didn't want Elena to freeze to death because of him, but Elena being stubborn stayed out with him, insisting he use the blanket.
And ever since than, he knew he wanted to spend the rest of his life with her, he somehow knew she would always be there for him, sticking by his side no matter what.
But that all changed, because of his and his little brother's desire to see the wolf's change, it was because of him. Klaus. He knew Henrik's death was his fault, and even if his siblings say they did not blame him, Klaus knew they each all secretly did.
Expect for Elena.
Elena was there for him, even when everyone else had shut him out. The only person who seemed to notice this was Niklaus's older brother Finn.
Surprisingly enough, Finn really did care for all and every one of his siblings, Finn was also an observer and over the years he saw how close Klaus and Elena had grew. Finn saw how much they love each other, he saw how loyal Elena had been, he saw her kindness shown to Niklaus, he saw everything.
They all said that Finn was a follower, always following his mother and her commands and wishes. This lead to believe that Finn didn't know anything that went on or that he just wasn't interested in whatever was happening. The complete opposite actually, and that was Finn Mikelson's greatest trait.
So when Finn noticed that his mother was being distant and didn't need him as much, he knew something was up.
Finn's suspicions were confirmed when their father killed them and they all woke up hungry for blood. Making them the first ever born vampires. The originals.
In those first few weeks of when they had turned, Elena grew worried. She had no idea of what they become, just that Esther had came to Elena saying that Niklaus and his siblings had grown sick and she needed her blood for a spell to cure them.
No one was a stranger when it came to magic. They all knew that Esther was a witch. So of course, Elena given it to her, praying that Nik would get better. But when she saw less of them, Elena worried. But her worries were over when Klaus came out again and shown his face.
He had went back to himself, expect he wasn't himself. Klaus had kept him turning a secret from Elena, which pained him knowing that the one person he trusted, he couldn't tell.
His emotions were heightened so of course he was overly sensitive. But that also meant his love and passion for Elena had grew. But even after all that he still couldn't get the courage to ask her if she would court him.