Don't Ever Get Too Comfortable
The biggest mistake is to assume that your life is going well from your own doing. You shouldn't even assume that this is God's way of rewarding you. And you certainly should never see someone else's life going better than yours and assume that this person may be spiritually superior to you simply because of the outward favors that God has bestowed upon him or her.
Understand that God's blessings work in complex and humbling ways. If He is showering you with His blessings, be sure to pass this test. Can you be grateful to Him, or will you fall into the arrogant assumption that you are somehow more worthy of this goodness than others or that it has come solely from your own efforts?
Can you realize that He can take all your happiness away in a nanosecond, and--more importantly--can your worries lie more in the notion that He may not have saved any blessings for your afterlife because you have consumed it all in this world than in your fear that the gold of now may disappear in an instant? If you can, you are spiritually mature.
Sometimes, God's blessings in this life is a sign that He has turned his back on us when we are sinful. When God loves a sinner, He will wake them up by tightening the reigns of worldly happiness just long enough for them to turn a brokenhearted prayer back to Him and straighten themselves out. And we are all hopeless sinners.
Don't ever stop repenting. Don't let a good life be the pitfall that makes you lose your humility. You're not living well because you are better than anyone else, or because you are more worthy. You are living well because this is His test for you. Will you pass the test?
I pray that He will shower us all with His goodness in this life and the Next, and I pray that He will protect us from our own evil. Most importantly, I pray that this world and the beauties discovered and enjoyed in it will never distract us from the Essence of Our Creator. Amen.