There will always be obstacles and challenges that stand in your way. Building mental strength will help you develop resilience to those potential hazards so you can continue on your journey to success.
There is a lot of research which suggests that resilient people are happier, and happy people are more resilient. It does, therefore, make sense to do what we can to build resilience as not only will this increase our levels of happiness, it will also improve our ability to deal with adversity. And just imagine what it's like to face challenges knowing that you have the inner strength to overcome, with success.