Whatever you want to call it: addiction, compulsion, habit, need, hobby, etc, the fact remains that it all has to do with brain chemistry and how you have wired your brains.
When any of us engage in pleasurable or rewarding behavior, our brains release the neurotransmitter dopamine, which acts as a chemical messenger between neurons and causes these neurons to form connections. Dopamine has many functions including effects in behavior and cognition, attention, motivation and reward, mood, sleep, and learning, but it is primarily associated with pleasure and reward.
Dopamine is released when the brain is expecting a reward. When you come to associate a certain activity with pleasure, mere anticipation is enough to raise dopamine levels. It’s a cycle of motivation, reward, and reinforcement. Read more about how it works here, and how the absence of the reward can cause you to want it (whatever it is) even more.
https://www.healthline.com/health/dopamine-effects
So basically, the more you crossdress and experience pleasure from it (especially sexual pleasure when you first started out), the more you will want to crossdress.
Eventually for many of you the sexual aspect abates, but it has definitely become a fundamental part of what you want to do. Some people want to call this a gender identity, either because they have no better explanation or because they feel it is more socially acceptable to explain it that way, or through a lot of reinforcement from communities such as this one. I’m reluctant to call it that, since I think it is entirely self-taught by the choices you have made - unless of course you are intersex (your chromosomes are something different than the standard 46xy for males, or 46xx for females), or you are one of the very few people who know from a tender age that they are in the wrong body. It’s easy as an adult to go back and redefine an original interest based on physical feel-good activities as a different gender ID.
Anyway, whatever you want to call it: addiction, compulsion, habit, need, hobby, it really doesn’t bother anyone. The reality is that after all these years, it has become something that is nearly impossible to eradicate. This causes no issues for most of you who are in relationships with the GGs (or GMs) who are OK with it. But it is definitely a problem for those of you in relationships with people who cannot live with it.
So for those of you who are experiencing issues, a therapist can help you achieve self-acceptance and balance if you want to stay in a relationship with someone who isn't keen on it. But, your partner would also have to learn to be flexible.