The First Noble Truth is "All things are suffering." Life is suffering. The word in Sanskrit is duhkha. Sometimes it is translated as "unsatisfactory". Situations, this world and our lives are not what we want them to be.
There's almost always a gap between what we want and what is. As much as we cannot accept that gap, we suffer. Our inability to accept life as it is and want things to be different, creates the suffering in our lives.
Zen Master Bon Soeng