The 19-year-old model was first romantically linked to the 21-year-old singer last year, with the pair confirming their romance during a New Year's holiday.
Following their vacation, it was alleged that Justin had been hooking up with Kourtney Kardashian - something Hailey was not impressed about.
A source said: "She demanded he stop stringing her along. She said he would lose her if he didn't man up and show he was serious about her."
Apparently it was Hailey's forceful attitude that proved so impressive to Justin, and he decided to give the relationship a chance.
The source added to Us Weekly: "Justin has never had a woman act like that before.
"He couldn't say no. So he's been seeing her again and is with her all the time."
Justin and Hailey been friends for several years and their families have given the relationship their blessing.
A source told E! News earlier this month: "Hailey and Justin have a very tight relationship. They both are very close with each other's families and care very deeply about each other.
"Hailey's family has been there and supported Justin through his transition so Justin has a strong respect for them.
"Both of their families love them together."