Use the mind to contemplate the body so that you’re familiar with it. When you’re familiar with it, you’ll see that it’s not for sure—every part of it is inconstant. When you see in this way, your mind will give rise to a sense of disenchantment—disenchanted with the body and mind because they’re not for sure, they’re unreliable. So you want to find a way out, a way to gain release from suffering and stress.
It’s like a bird in a cage: It sees the drawbacks of not being able to fly anywhere, so its mind is obsessed with finding a way out of the cage. It’s fed up with the cage where it lives. Even if you give it food to eat, it’s still not happy, because it’s fed up with the cage where it’s imprisoned.
The same with our heart: When it sees the drawbacks of the inconstancy, stressfulness, and not-selfness of physical and mental phenomena, it will try to contemplate how to escape from that cycle of wandering-on.