Why do dreams always stop before an event?

Last night i had this really intersting dream that i had to break into the cellar of a bank and the point of entry was through my mind...there was a living machine down there that talked...and if my heartbeat slowed down too much i would die. It was like a cross between Inception,knightrider and Crank. Anyway back on topic, just as i was opening the safe and about to find out what was inside i woke up. Why does this always happen? I often find when my dream is about to climax to an interesting conclusion it ends abruptly