Correct technique is vital when boxing and doing weights for the health of your shoulder if you have problems. Incorrect technique when benchpressing for example puts pressur on the ligaments in your shoulder and you can develope something an old gym instructor told me called loose shoulders, where the ligaments have been stretched making the joint more vulnerable to dislocation. I don't actually bother with heavy weights any more, but I do some strengthening using resistance bands and light weights. Strengthening the muscles around a joint will take a little of the burden off the ligaments themselves and usually helps, but you need to make sure you retain the suppleness of the muscles with stretching, otherwise they will become susceptible to injury themselves opening a whole new can of worms.