This is an interesting question. I would say that, for me, assertiveness is a pretty core part of my adult personality but it has definitely grown, more with aging and life experience than with sobriety per se. Where sobriety makes a huge difference is choosing more wisely what I engage in, what I stand up to, what I tend to push... and how. Not necessarily in quantity, more in quality. I think drinking often makes people aggressive, it does so via affecting how the brain works, including loosening inhibitions, but that is rarely a healthy, constructive form of assertiveness. Alcoholism makes people painfully insecure even if they do not admit it and insecurity/aggression are intimately linked. I think drunks are rarely good communicators and a key element of becoming more assertive is being able to communicate effectively with a variety of people. Good recovery usually also increases our self-esteem and -confidence and that's very important for assertiveness as well. So I think for me it is sober maturity. I wasn't nearly as assertive as a young person without a drinking problem.