Mac doesn't think about what has to happen, instead, he focuses on the last time he saw Toby's grin, and how alive she was. If Cross never starts down his road, Mac will never meet Cross, and will certainly never meet Toby--and even though he is leaving her behind, she is alive, and if he doesn't leave--well, that's not a chance he's willing to take.

Crossing through the anomaly, he listens; in the distance is the roar from the T-Rex, and Mac smiles a little, as he knows the circle is about to complete, time has a way of catching up with you and he's not fighting it anymore.