Supposedly, imprinting is supposed to have the outcome of passing wolf genes most effectively. But when Leah phased, that went into question. Because she stopped having her menstrual, the whole thought of reproduction purposes caused question between imprints and their mates. It all came down to two things in Leah's mind:

Actual Soulmates

Or

It was fate testing the strength of the wolves

It was supposed to be rare. That cause a theory in Jacob's mind:

A lot more wolves were going to be needed to kill leeches in the future

But Leah had wanted kids. Even thought they would never be with her first love, she craved to have a family of her own and be happy. Another problem came to her mind though:

Would she not be able to have kids until she found her imprint?

Would she ever find her imprint?

Why does fate keep intervening with her plans to move on with her life?

She came to the conclusion that her imprint was in the reservation since everytime she prepared to leave, fate intervened. She was going to marry Sam, nope, fate has him imprint on her cousin. She is ready to move on with her life and leave the reservation, no-no, fate gives her father a heart attack and she phases into a wolf for the first time. Thus, making her permanently connected to the place that has caused her enough pain over the years.

Jacob didn't ever want to imprint. He felt like he would be losing his independence like everyone who had imprinted has. He did not want to have to be forced to love someone. Imprinting was not love at first sight in both his eyes and Leah's. His had everything he needed to ground him. His father. His sisters. His pack. And his friends. An imprint wouldn't be necessary to keep him grounded to the pack. If anything, it causes a wegde. Once someone imprints, their imprint is the most important thing above any other life. The pack is supposed to be the defender of humans. Imprinting puts that second.

When Jacob imprinted on Renessmee Cullen, he felt everything that mattered to him, that tethered him, break. All that remained in the end was his tie to her. He hated it, but the imprint forced him to care about the hybrid child. He had no choice or say in the matter. In fact, just before he had imprinted, he wanted to take off with his pack to Canada. He wanted to build his own life away from all the leech drama and bad memories. But he could do none of that now. He was miserable and just as he predicted, the imprint made him feel happy about. He was no longer Jacob Black, True Alpha of La Push. He was the future of Renessmee Cullen.