Jiayun, this is a really insightful thought!
There might be a variety of answers to your question depending on the specifics of why an individual is struggling to be content with God, but here are a couple of ideas that are swirling around in my head.
Sometimes we struggle to be content with God because we are trying to satisfy our desires with other things. Earlier today our son, Raphael, was desperate for his bottle, but we were having trouble getting him to feed because he was so convinced that was he needed was his pacifier. We would offer the bottle to him, which is what he really wanted and needed, but he was so focussed on the imitation that he was missing the real thing! (I owe this observation to Elizabeth Natirboff; thank you for caring for Raphael today! :-)) Often times we do the same with God. We don’t allow him his full and proper place in our lives; it should be no surprise, then, that we find ourselves disconnect.
A second idea that I’ve been reflecting on recently, related to your question, is that currently we see only a reflection; one day we will see face to face. Therefore, sometimes we are not content with our current experience of God precisely because we were made for a still greater experience of him, one that still awaits. I was doing some research on separation anxiety recently. One definition of this condition is “anxiety regarding separation from home and/or from people to whom the individual has a strong emotional attachment.” Anxiety is one form of discontentment, and it should be expected that Christians can be vulnerable to this type of anxiety. After all, the Bible tells us that we are not home; we are foreigners and exiles, and our citizenship is in heaven. Plus we are currently separated in some respects from the full face to face communion with God that we were made for. CS Lewis says we have trouble imagining a holiday at the beach when all we have known is playing in mud. The flip side of that intellectual coin is that once we have tasted a holiday at the beach (i.e., the depth of communion with God we were made for), it can be hard to find contentment living in the mud of our broken world.
This is the already/not yet nature of Christian faith, but I take comfort in the fact that God is not expecting me to be fully content with my current state. He wants me to desire more, and to desire it more strongly, and to pursue it more passionately. And he has not only promised that there is more, but he made good on that promise in the proof of the resurrection.
By the way, on this topic I would recommend the book “More” by Simon Ponsonby. God bless you!