Preaching on Thanksgiving

How do you go about preaching for an ecumenical thanksgiving service? I guess I'll find out cause I'm doing that soon. I think my time limit is about 12 minutes. That alone creates an almost impossible challenge. I do have many thoughts and questions. Mabye I'll get some answers.

1 Thessalonians 5:18

16Be joyful always; 17pray continually; 18give thanks in all circumstances, for this is God's will for you in Christ Jesus.

Why are we to be thankful? What's at the core of thankfulness? Thankfulness alone doesn't seem helpful.
Why did 9 out of 10 lepers not thank Jesus?
Does the historical reason for thanksgiving matter during a Church thanksgiving service?
How do I keep from being extremely cynical about such a service (ecumenical)?

Let me end this post with the words of one of my favorite hymns - as far as simple words go (by the way, Blessed Be the name of the Lord seems to be a new version of the same idea). Thanks to God for My Redeemer

August Ludvig Storm

(1) Thanks to God for my Redeemer,
Thanks for all Thou dost provide!
Thanks for times now but a memory,
Thanks for Jesus by my side!
Thanks for pleasant, balmy springtime,
Thanks for dark and dreary fall!
Thanks for tears by now forgotten,
Thanks for peace within my soul!

(2) Thanks for prayers that Thou hast answered,
Thanks for what Thou dost deny!
Thanks for storms that I have weathered,
Thanks for all Thou dost supply!
Thanks for pain and thanks for pleasure,
Thanks for comfort in despair!
Thanks for grace the none can measure,
Thanks for love beyond conpare!

(3) Thanks for roses by the wayside,
Thanks for thorns their stems contain!
Thanks for home and thanks for fireside,
Thanks for hope, that sweet refrain!
Thanks for joy and thanks for sorrow,
Thanks for heav'nly peace with Thee!
Thanks for hope in the tomorrow,
Thanks thru all eternity!