Whilst I agree that, yes, doing other things aside from focusing on your transition is healthy and the way to a happy head, I'm not entirely convinced by the advice to 'find new interests'.
Personally, and I accept this may be wildly different person to person, I'd already built a life that I enjoyed because life is to be enjoyed. I got into things I like, hung out with people I like and went to places I enjoy. Once I started aligning my body more effectively with my brain I didn't just up and leave that life as all those things were still massively important to me as a person and to my happiness.
Particular activities aren't solely 'male' or 'female' pursuits and I see no reason why, if you like going to raves, or fixing cars, or electronics, or going to the theatre, or running, or fighting robots or whatever, that you can't continue to do these things during and after your transition?
I often think that the 'I was this and now I'm this' approach that is often advocated can, potentially, be quite damaging as the added pressure of, effectively, starting a new life on top of everything else, which has the symptom of cutting off any support networks that have been built up, must add more anxiety and stress to an already pretty heavy situation.
I'd imagine, for some people, those around them may be toxic enough to make this plan of action a good move and everyone has a different situation that they are operating within so nothing is black & white. I do feel that some trans folk, either due to the community line of thought or because therapists think this is the only way to go about things, just assume they have to cut all ties and make a new start when this, as many people can no doubt attest to, is definitely not the case.
Obviously, everyone's mileage will vary wildly, this is just my 2 pence worth, for what it's worth, as it's a subject that's been bothering me a bit as my transition has progressed.