Thanksgiving is the best American tradition. It is one of the traditions supported the most by the Scripture.
For Things That Are Given to Us
We give thanks because we are so blessed by grace in light of our situation. At this age, people tend to think that they deserve what they have because of their hard work. While we are called to study hard at school, to work hard in our vocations, or to serve Him faithfully and diligently, we have to recognize that He is the One who ultimately provides.
Provision comes through other communities (as in Greek Christians sending donations to Jerusalem, as in 1 Corinthians 16:3), through naturalistic means (Hebrew 1:2-3; Matthew 5:45), or through other ways. Very often people do not appreciate this until they run into a risk, as in the story that while Abraham was about to sacrifice his son, a ram was placed at the altar (Genesis 22:12-13).
We give thanks for His grace. We are called righteous because of His unconditional election (Romans 3-5) and His redemptive work done through the history. We do not deserve it, but we are forgiven, as in the pardoned turkey at the White House.
And sadly there is another turkey not pardoned as it is not chosen. That is our original destiny. Even President Bartlet could not pardon it.
To Counter Against Human Nature of Ungratefulness
The human nature is that we tend to take things for granted. God knows we are like this (Romans 3:23). Throughout the history of ancient Israel is the ongoing disobedience and repentance. Even if we understand God’s command to give thanks, we often forget (Romans 7:21-23).
Therefore, the Israelites set up stones in memory of what God had done to them (Genesis 28:18; Joshua 4:19-24) lest they would forget. We celebrate thanksgiving lest we forget to give thanks.
As an Imperative
The Scripture is not lack of the teaching of thanksgiving.
 I will offer to you the sacrifice of thanksgiving
and call on the name of the LORD.
(Psalm 116:17 ESV)
 Rejoice always,  pray without ceasing,  give thanks in all circumstances; for this is the will of God in Christ Jesus for you.  Do not quench the Spirit.
(1 Thessalonians 5:16-19 ESV)
- John C Abell, “Daily Pulse: Thanksgiving Here, There and — in Space! Wanted: Commercial Pilots (No Experience Necessary)”, LinkedIn Pulse (Nov 25, 2015). [LinkedIn]
- J. M. Frame, “A Primer of Perspectivalism” (2012). [Link]