What Is the Primary Factor That Causes an Option’s Delta to Move Closer to 1 or -1?

The primary factor causing an option's Delta to approach 1 (for calls) or -1 (for puts) is the option moving deep Into-the-Money (ITM). As the underlying asset's price moves significantly past the strike price, the option behaves more like a direct position in the underlying asset.

For example, a deep ITM call option will almost certainly be exercised. Consequently, its price will move nearly dollar-for-dollar with the underlying asset.

This signifies a high probability of exercise and a high correlation to the spot price.