It is just is; the next thing accompanying the knowing of life as is, is the cause of separation.
The thought of resisting one thought or the other is the cause of separation.
The thought of resisting the present situation and not being able to accept the seemingly ordinary and banal course of things causes suffering.
The simple movement in compulsive loops of thought-identification causes suffering. It goes on as:
Thought →Impulse → Identification → Separation → Suffering
And until and unless one cuts the umbilical cord of thought-identification, one cannot escape the addictive cycle of suffering. Be wary of replacing one thought-identification with other, be wary of the seemingly refined identification because they too are identifications.