I don't lift heavy anymore, but I think I do know what you are talking about.
I always felt more agressive after a really hard workout. Some people call it that 'pumped' feeling, 'endorphin rush', 'Runners High', others have different names... it generally refers to feelings of exhilaration brought on by pain, danger, or other forms of stress.
"...when strenuous exercise takes a person over a threshold that activates endorphin production. Endorphins are released during long, continuous workouts, when the level of intensity is between moderate and high, and breathing is difficult. This also corresponds with the time that muscles use up their stored glycogen. Workouts that are most likely to produce endorphins include running, swimming, cross-country skiing, long distance rowing, bicycling, weight lifting, aerobics, or playing a sport such as basketball, football (soccer), or American football...
A study in 2004 by Georgia Tech found that runner's high might be caused by the release of another naturally produced chemical, the endocannabinoid anandamide. Anandamide is similar to the active chemical, THC, found in marijuana. The authors suggest that the body produces this chemical to deal with prolonged stress and pain from strenuous exercise, similar to the original theory involving endorphins. However, the release of anandamide was not reported with the cognitive effects of the runner’s high; this suggests that anandamide release may not be significantly related to runner's high." - wikipedia
I get them frequently, though not as much as when I was younger.