Ada was given orders by Umbrella to hunt you down and kill you for whatever reason. To increase her chances at succeeding, they decided to give her a highly-experimental serum that supposedly made tracking you down as easy as touching something you’ve touched. It would also increase her general strength, agility, and endurance by quite a bit.

The serum worked for the most part, if disregarding the side effect of her being now completely obsessed with you. The scientists that made the serum added oxytocin, and a lot of it. They thought it would make her more determined to get the job done. She isn’t determined to kill you now, she’s determined to make you feel what she feels for you for her.