Bruising, a common affliction, happens when we rupture blood vessels under the skin (usually by whacking a limb on something). This injury results in inflammation, which triggers pain receptors in the muscles and skin. I.e. it hurts. So why would we then poke or rub it while so sensitive? It's actually a way of managing the pain. When we rub a painful area, it triggers other receptors in the skin, such as those that sense pressure, light touch, temperature, etc. By activating these other receptors for a short time, it actually "drowns out" the pain being detected by the brain, providing a moment of relief. 

Picking scabs is an entirely different issue, but like rubbing a bruise, something we seem to do instinctively. On one hand, it can be a symptom of "obsessive-compulsive tendencies that often signal an underlying, often as-yet-undiagnosed mood or anxiety disorder." On the other, some experts say it's a remnant of the way our ancestors were compelled to groom, lick, and preen themselves. Scabs are like plaster over a healing wound, and they definitely itch, so maybe it's just a response to the fact that we're hyper aware of it.