Simply put, it's one that is able to adapt to the world and stress around it.
Let's deconstruct that.
Social: What we call picking the right tribe or establishing meaningful connections to those who are in your life circle. Studies show time and time again that this is a crucial piece to a long lasting and productive life.
Physical: One that allows for adequate levels of movement. This often gets interpreted as spending time in a gym but we're not referring to fitness, which can be an important part. The body is more interested in movement which may mean working in your garden, walking your dog or simply not being still for large chunks of the day. Can you perform variations of all the fundamental movements? Squatting, lunging, pushing, pulling, dead lifting, twisting, climbing, crawling and gait (walking running). When's the last time you crawled? :-)
Nutritive: Emphasizing real food and learning to be more attune to your bodily needs. What does that mean? Overriding what you've learned about food and nutrition and becoming aware of natural signals for certain foods, like hunger and its many stages.
Psychological: Allowing for a full spectrum of experiencing the world as it comes, being able to test beliefs and feel comfortable doing so. The mind is the most plastic tissue in the body so allowing it to freely flow without judgement is the healthiest place to be.
The better we can navigate our daily stressors, the greater chance we have at achieving enhanced health overall and the lean, fit body we were meant to have. You can easily break each of these down and see how they play a role in the weight gain process. Even the slightest shift or imbalance can significantly impede your efforts.
No one ever said fat loss is easy, but by understanding that our natural state is one of leanness you can see how the process might be simple.