I don't mean life is difficult because of difficult things - but that even it its best and smoothest and happiest and healthiest Life Is Difficult.
(If life isn't difficult, you aren't doing it properly.)
It is difficult essentially because each of us is unique and has a distinctive destiny which must first be located then lived.
That each of us is unique is not an achievement, nothing to be proud of, but simply in the nature of human beings: a datum.
However, it means that nobody can tell us exactly what we should be doing; so even when we are doing broadly the right kinds of things, we are still confronted with the details that need to be evaluated, then executed.
This is not something to complain about, however! It is the great adventure of mortal life, and all the better from its having no precedent and no possibility of being repeated.