Sirius during his school days at Hogwarts was actually kinda cocky. He was confident, with his looks and his charming personality, no one would be able to resist him. He never really had that much time to fall in love with people, true, he did like messing with people every now and then but he never expected to fall in love

You were in Sirius' year and he didn't really notice you till a certain amount of time passed. He was surprised, how come he didn't notice someone so cute looking like you? He observed you for some time: Your characteristics and your personality. He saw how kind you were and how sweet and shy you were, always kept yourself to yourself. He wondered how you even managed to end up in Gryffindor when you could've done well in Hufflepuff but whatever, he wasn't complaining. He had you closer to him and he was thrilled

His first goal was to become your friend and then ask you out to the Three Broomsticks where he'll confess his love to you and the both of you would be a happy lovely couple. It was the perfect plan for him, till you decided to form another path for yourself

You couldn't deny that Sirius was handsome and charming. But you weren't really looking forward to be in a relationship with anyone at the moment. So, when Sirius called you to the Three Broomsticks, he handed you a rose and ordered butterbeers and some desserts for you guys to eat. He didn't give you the idea that he was confessing his love for you since he wanted it to be a surprise, a surprise which of course his friends James, Lupin and Peter helped him in. They helped him in the decorations and keeping his bratty little brother Regulus away out of sight so he couldn't steal you from their friend

After you were done talking and chilling with your drinks and desserts, Sirius held your petite soft hands in his rough and large ones and went "Y/N, uhh... I don't know how to say this but I really love you and I'd love it if you went out with me'' he said blushing. Your heart dropped, you most certainly weren't expecting this. You just thought he invited you to have a chat for old times' sake and to relax as friends, you had no idea he was going to pour out his feelings for you

You gulped nervously and a heavy feeling settled in your chest. You didn't want to let him down, you REALLY didn't want to, but you had to follow your heart. And right now your heart was telling you to live life on your own terms and not to be in a relationship with anyone. Besides, you just befriended Sirius only for a few months, you had to get to know someone properly for like, YEARS before you wanted to date someone. You tried to let him down gently "Sirius, you're a great guy. I'm sure you have lots of people fawning over you and people who'd die to be with you, but.... how do I say this....'' you trailed off and rubbed the back of your neck looking awkward and a bit sad. "Tell me what?" asked Sirius anxiously peering at your face. He didn't want to hear your answer anymore, it would break his heart and he wouldn't be able to handle it

"I'm really not looking to be in a relationship with anyone right now... I'm sorry, I'm so sorry....'' you whispered and stared down at the table, not wanting to meet his eyes. Sirius felt numb, he felt like his heart was shattering into a million pieces and he couldn't move. You told him a quick 'bye' and ran off, his friends ran to comfort him

Meanwhile after a few days, you tried forgetting about him. But the hurt look on his face kept haunting you at the night. You were in the courtyard reading a book. The weather was nice and windy, quite the opposite of what you felt. You felt down in the dumps for rejecting Sirius, but you had a feeling he'd most probably forgotten about the whole thing anyway and besides, he could always find someone else. You had no idea how wrong you were: Sirius was not able to forget about you, he confronted his brother Regulus and asked him if he had anything to do with you rejecting him. Of course he ended up jinxing Regulus in anger since he felt that he was lying. Next he bullied Snape for a while thinking he manipulated you or bullied you to reject him till you intervened and asked him to leave him alone

Sirius was infuriated and it was like rubbing salt in a wound. Why TF were you standing up to SNIVELLUS SNAPE? Maybe he was right... maybe Snape HAD something and his worst fears were now confirmed. You could be dating Snape. His friends found out that you weren't dating anyone and that made him relax a little. He stalked you in his Animagus form for a while and you of course didn't know it was Sirius in Animagus form. You thought it was just a random cute dog and you spent time with it. Every single second you spend with him, holding it and cuddling with it, you were fueling his dark twisted desires for you. And these days you could see that Sirius was looking happier for some reason so... you assumed it was all good for him

It was... since he was planning to kidnap you the next time he turned into his Animagus form. Not exactly 'kidnap' you per se, he'd just keep you in the Shrieking Shack, but don't worry, Sirius won't leave you alone. Your kindness and sweet nature was too addicting for him and he was far too selfish to share you with anyone else....