People often say to my spouse about her transition, "Whatever makes you happy." Some say it with a dismissive passive-aggressive tone. Others say it with a warmhearted caring tone. We wish they wouldn't say it at all.
Changing genders isn't about the pursuit of happiness; it's about survival.
Finding a better job makes you happy. Getting a new wardrobe makes you happy. Adopting a pet makes you happy... These are lifestyle choices.
Gender change is a medical necessity to endure life. If it weren't, suicide rates among trans people wouldn't be so high, and transition wouldn't include medical treatments covered by insurance.
You wouldn't tell a cancer patient that you support her decision to undergo chemo because "whatever makes you happy." The end-result may be a happier life free from cancer, but the treatment is miserable. Likewise, transition is a long and painful process to free the patient from gender dysphoria. Nobody makes the decision lightly.
"Whatever makes you happy" demonstrates a lack of understanding about the gravity of the transgender condition and the challenges of transition. Please don't say it. There are better ways to show your support.