Scott - Peter
When Scott first got turned by the big bad Alpha Peter Hale he was scared and then he found his 'wings' or claws to be exact. After he turned Peter pack offer down Peter came to the school dance and instead of turning just Lydia he turned both Lydia and you. Peter turned you out of spite he thought you were weak and assumed if he gave you the bite you would die and then he could pressure Scott into his pack if he promised to bring you back to life.
Stiles - Aiden
When Aiden was an Alpha and still alive he turned you out of jealousy. He thought that if you were a werewolf and turned by him you'd have this bond and be almost sired to him. He thought that this sire would lead you to break up with Stiles and become his girlfriend like he always wanted. But luckily for you there was no sire bond and you and Stiles stayed together leaving a very jealous and angry Aiden behind.
Isaac - Deucalion
You were with Isaac at Derek's loft and then next thing you know Isaac is being held down by Kali and Deucalion has his fangs in you. You didn't know it then but the reason that he turned you is because he knew your parents (who were werwolves) when they were alive and only wanted to do right by them by turning you so you could carry on their 'work'.
Derek - Derek
Derek turned you because he knew that if he did you could protect yourself and also so that you could be together fully. You didn't ask him to turn you but deep down you wanted him to so that your relationship could progress. You knew it would be hard and so did Derek but he knew he could help train you and you knew he could to.