"It takes strength of character to feel genuine joy and excitement for other people's success. Mentally strong people have this ability. They don't become jealous or resentful when others succeed, although they may take close notes on what the individual did well. They are willing to work hard for their own chances at success without relying on shortcuts"
Mentally strong people enjoy and even treasure the time they spend alone, They use their downtime to reflect, to plan, and to be productive. Most importantly they don't depend on others to shore up their happiness and moods. They can be happy with others and they can be happy alone.