Essentially, people suffer due to a general inability to heal or otherwise get over trauma of whatever physical, mental or spiritual definition. You'll suffer from physical ailments until such time as they're healed. You'll suffer from mental ailments until they are found and at least reasonably resolved or otherwise come to tearms with. Likewise for ailments of ones spiritual beliefs. Resolution for spiritual problems pretty much have to be found from within.
Suffering takes on many forms. Some people suffer by whichever cause and barely show it. Others with the same or similar affliction may be nearly incompassitated if not obviously hindered. Is it a testiment to the perseverance of ones personal will that seperates the two? Perhaps, at least in the case of physical suffering, it could be attributed to a superior pain tolerance. And I suppose mentally, there are greater and lesser tolerances for pain, too. But why do we feel any sense of suffering in the first place? Life would seem to be much easier if we didn't.
Physically, suffering is usually due to a persistent pain or the physical effects left by disease or other trauma to the body. Each time the body part or function hindered by said is utilized, one is reminded in an obvious way of its limitations and thereby, the affliction itself. Perhaps this mechanism is there to prevent us from injuring ourselves further.
Mentally, suffering is often an even nastier beast, as there aren't usually operations or procedures that can correct it. It can be very difficult to even define what the specific problem is in many cases, as we seem to add all feelings of disappointment and regret into the same pile. Once mental suffering begins, it seems to be very deeply seeded in our psychies and lower our overall mental resilience across the board. If no actual, chronic chemical imbalance is the cause of the suffering, little more than time is typically needed to heal the issue.
Strangely, sympathy from others will usually help speed up the process of healing mental suffering caused by circumstance (as opposed to chemical imbalance), to an extent. Too much sympathy can actually cause feelings of suffering to drag on.
We are social animals. The loss of a loved one is pretty much the greatest amount of mental suffering one can take on. This is because we know what 'gone forever' means. It's an absolute that we can't do anything at all to prevent or change. When a loved one is taken from us, it can really feel like a small piece of us, somewhere deep inside our mortal core, dies too. It's our longing for that which we love but can no longer have. It may be that too much sympathy can be adopted in a general sense, to fill that hole with similar feelings of caring and love. In that case we can see how one may become dependent on the sympathy offered by those around us.