What helps you grow stronger physically, emotionally, mentally or even spiritually?
In one word?
Then consequent adaptation.
People have this firm belief that all 'stress' = Bad. But stress is merely a force acting upon you. Otherwise known as a stimulus. This could be expressed as 'being outside of your comfort zone' or dealing with 'change.'
However, an easy argument for the proliferation of the human race exists that human beings are among the most adaptable organisms on the planet. Our creativity and ingenuity has allowed us to thrive on every continent in nearly any environment, even really harsh ones like the Arctic Circle, Antarctica, The Plains of Africa, The Outback, The Sohara and dense jungles like the Amazon.
Ultimately it's adaptation to stress that determines how we advance as people, the same way a plant generally grows towards the stress of sunshine, or adapts to the chemical makeup of the soil it's in, or responds to rain.
The key here is that if the stress is too high, we adapt poorly；if the stress is too low, we don't adapt at all. The stress needs to be just right and it needs to be applied deliberately over a period of time to yield an adaptation.
Often one dose of stress is not enough to force continued adaptation either. So continued development requires steady progression as well.
For instance. If you want to get good at math, you need to practice math problems that are continually challenging enough to make you really think about the answer. Once you understand the underlying concepts to the answer of 3+3 to a point of automaticity, you have to eventually practice more complex problems like 5 + 3, or 3 + 3 + 3. If you don't slightly over-reach on ability constantly, you get stagnant progress and thus 'do not grow stronger.
Bottomline: You can't have adaptation without an appropriate stimulus.