Although he had become Earth's most powerful sorcerer, Strange was not happy, because even as the Sorcerer Supreme, he could not revive Christine unless he used the Time Stone to return to the past.
The problem was that this was the greatest of taboos. One careless mistake could bring about the destruction of the world. As the Sorcerer Supreme who protected the world, he could not do such a thing.
"There's clearly a way, yet I can't save Christine."
This situation filled Strange with pain and struggle every day. On one lonely night, he finally could not bear it any longer and used the Time Stone to return to the past.
This time, Strange drove carefully, but the car still overturned, and Christine died just as she had last time.
Unwilling to accept this, Strange continued traveling through time. This time, he took a different route, but it still ended in a car accident.
The next time, Strange simply did not attend the banquet. Who would have thought that a car would crash into the café and take Christine's life?
The time after that, Christine was poisoned. The next time, the building she was in collapsed. The time after that, she was shot during a robbery...
No matter how many times Strange traveled through time, he could not change Christine's fate of death, as if it had been predetermined.
"Why is this happening?"
After hundreds of attempts, Strange knelt at the scene of the car accident and wept in utter despair.
"This is an Absolute Point."
The Ancient One from that point in time appeared before the despairing Strange and said, "If Christine does not die, you will not learn magic. If you do not learn magic, you cannot return to save her. This is a time paradox.
"Strange, stop.