Rationale for 1. It's widely supported by the medical community and by scientific studies, and helps improve the quality of life of people with gender dysphoria.
Rationale for 2. Gatekeeping traditionally consist in telling a psychiatrist anecdotal personal stories about your gender expressions and behaviors. Some psychiatrist hold sexist attitudes, and can prolong your gatekeeping or refuse treatment if you don't tell them a nice tradwife story (always known, only attracted to boys (if MtF) or girls (if FtM), always played with [gendered toy]).
This even has the opposite effect of not wanting to talk with them about genuine doubts, since they can deny you HRT if they don't like your adherence to sex stereotypes.
In the end the trans person themself is more qualified to know whether they would like their bodies to be more masculine or feminine.
(EDIT: added rationales)