Also, one person's pain is simply not comparable to another person's -- even if the situation is exactly the same, it will affect people in different ways because we are all so different. If someone walked up to me and cussed me out, insulting my looks, my intelligence, my morality, it wouldn't wound me deeply, simply because that is not a weak point for me. But if it happened to someone who was extremely sensitive about their looks or intelligence or morality, it would be devastating. The exact same action would inflict far more pain on one person than another. And all situations are that way! We can never never never understand another person fully, because we have not lived their life and we do not have their exact personality, so we can never make assumptions about how painful something may be. No one ever has a right to say, "my pain is greater than yours," or even "so-n-so's pain is greater than yours." Some wounds are by nature more destructive than others, true -- sexual abuse creates more lasting devastation than an insult, for instance -- but that does not invalidate the 'lesser' pain. Both people are deserving of equal sympathy and support. One person may need more than another, but they are worth the same, and neither should ignore her/his own pain to focus on another's. Heal yourself first so that you can see clearly to help others heal.
All of that said, there is a difference between expressing pain and merely complaining -- the difference is that expressing pain deals with a wound, whereas complaining deals with something that is not really hurtful, but instead offensive/irritating. So many times I have been irritated with a customer (where the only thing that was hurt was my pride) and SO tempted to come home and blast her/him in my journal, (and I have yeilded sometimes!) but I really believe that to do so would merely add negativity to negativity. Pain may be negative, but if your goal is healing or growth, pain is also positive. Offense/irritation are never positive, and dwelling on them simply feeds the negativity. It's the difference between picking at a scab (useless and destructive) and getting surgery (painful but productive).