How do you become less aggressive?

One way is practice.

Aggressive behaviour, if your like myself can result from feelings such as not being understood or not being listened to.

If this is the case, becoming more affective at communicating can REALLY help. There are really simple steps you can take at becoming a better communicator if you google around, and if you want to be a serious communicator, look at thinks like NLP (Nuero Linguistic Programming)