Jake was on a patrol, he smelled Sam in the area, and knew he was probably on the same mission. This was all ridiculous and he hoped they'd figure it out one day. He didn't see that day coming anytime soon, but it didn't stop him from hoping for it.
Maria had warned him (vaguely) and then Alice had warned him a bit more honestly about what was coming. Jake wasn't thrilled, but he was willing to protect what was his. His land, his family, his friends, his Maria, God help the creature that threatened any of that.
The wind changed and he caught a smell of an unfamiliar vampire in the area and immediately his ears went forward and he bared his teeth as he started running in the direction of the offending vampire.