Right here in cyclic existence, where we have been throughout all previous time without beginning, we have been holding on to our own deluded projections, thinking them to be true and lasting. This is precisely what serves as the cause of our wandering in this circle of samsara.  

That is why mindfulness of impermanence is so crucial.

To develop this, you need to get it straight in your mind that as soon as you were born, death began approaching step by step and will most certainly catch up with you... Right now death is also drawing closer and closer with each passing instant.  

Sentient beings are not aware of this impermanence but set their hopes on lasting and count on continuing to be here moment by moment. Nevertheless, they all pass away; suffering is inevitable... This is how things are [in samsara].  

What you can do is be aware of it and through this develop renunciation [of samsaric existence].