The popularity of the Life is Good product lines can, I think, be rightly read as something of a backlash against the cynicism, negativism, pessimism, and even hatred, that seems to fill so much of contemporary culture. You can see these latter qualities in TV commercials that celebrate selfishness, in reality shows that elevate narcissism and nastiness, in movies that don't blink at showcasing dehumanizing coarseness, and even in the refusal of schools to show their students a "stay in school" message from the President of the United States.
But life is good. Not because T-shirts say it is. Not because there aren't bad, sad, even tragic things that happen. And not because there aren't cynical people trying to shaft others.
Life is good because the God Who made us hasn't given up on us and, so long as we never give up on Christ, never will. Please read this passage from the New Testament (which I've told my wife must be read at my funeral or else I'm popping out of the box to read it myself), then go read this outstanding reflection by Julie Ackerman Link:
What then are we to say about these things? If God is for us, who is against us? 32He who did not withhold his own Son, but gave him up for all of us, will he not with him also give us everything else? 33Who will bring any charge against God’s elect? It is God who justifies. 34Who is to condemn? It is Christ Jesus, who died, yes, who was raised, who is at the right hand of God, who indeed intercedes for us. 35Who will separate us from the love of Christ? Will hardship, or distress, or persecution, or famine, or nakedness, or peril, or sword? 36As it is written, “For your sake we are being killed all day long; we are accounted as sheep to be slaughtered.” 37No, in all these things we are more than conquerors through him who loved us. 38For I am convinced that neither death, nor life, nor angels, nor rulers, nor things present, nor things to come, nor powers, 39nor height, nor depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord.Life is good. Amen!