What causes a wound to weep?

Part of healing. When there is injury to our skin, inflammation occurs which means the area becomes red, warm, tender and with swelling. The swelling occurs under the skin layers and then can leak out of an open wound. The fluid contains water, protein, electrolytes and cells that give it color. This is a natural process and aids in healing. As it heals, the drainage will decrease. Keep it covered.
Swelling. Not a lot of information from your question. Going to presume the weeping is clear liquid in which case it's likely that the wound is near a swollen limb such as a venous ulcer of the leg. Need to control the swelling with a compressive bandage to affect healing.
Healing. The body will naturally try to heal itself. Swelling is a normal reaction by the body, but excessive swelling (weeping) could be an infection. Let your doctor know if you have concerns. Best of luck.