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Loki and Thor met for the first time at the latter’s twelfth birthday. Odin and Frigga had been dating for several months at the time and decided that was the perfect opportunity to introduce their two sons. As single parents, they feared a lot this encounter, the boys were so different that it was easy to imagine them disliking each other. In fact, immediately after being introduced, they went separate ways and ignored the other during all the party. But then, as the guests went away they found themselves alone and started talking they somewhat connected and then became inseparable.
By the time Thor’s thirteenth birthday came they were inseparable. It amazed everyone how their friendship worked. They could either spend their days indoors quietly reading to each other or outdoors playing all sorts of games. Thor was very affectionate with Loki and protective when he was bullied by the other kids, while the younger one was subtler and showed cautiously how much he cared about the other. It seemed like they had started to consider each other as brothers, so nobody was really surprised when Thor kind of announced that he wanted to spend that day alone with Loki.
Then, as Thor turned fourteenth, Frigga and Odin threw a surprise party for him. They made it big and needed all an afternoon to set the thing up, so Loki distracted and stayed with his new stepbrother for as long as he could. That night Thor realized that, even if he had appreciated the party and all the presents that his friends had given him, the best part of the day had been the one spent alone with Loki. The younger boy seemed to have become “his favourite person”; but that was only legit since they were now related, right?
The next year Loki himself organized the celebrations for the birthday. He planned the best day he could think of and, even if he hated it, he also let Thor’s friends help him with it. All went smoothly and at the end of the day Loki was proud of himself. He didn’t exactly have fun, being around so many people for so long always tired him, but Thor did and that was his goal after all. So, when his “brother” hugged him close and whispered, “Thank you” into his ear he couldn’t help but think it had been worth it.
But the best birthday Thor ever had had been his sixteenth. He always said that it was because that night he finally “opened his eyes”. The party his friends had threw him was perfect and he was having lots of fun till he saw Loki with some girl in the kitchen. She was clearly flirting with him and that made Thor incredibly jealous. He felt the need to have Loki’s attention so deep he just went there, grabbed his step-brother, and kissed him. And as the other kissed him back he realised he had been in love all this time.