And God spoke all these words, saying, 

“I am the Lord your God, who brought you out of the land of Egypt, out of the house of slavery. 

“You shall have no other gods before me.

“You shall not make for yourself a carved image..." etc. etc. 

Exodus 20:1-4 ESV

You should recognize those last two verses: the first and second of the Ten Commandments.  If I raised you right, you know most of the others: Don't take the Lord's name in vain; keep the Sabbath holy, honor your father and mother, thou shalt not murder, adulter, steal, bear false witness, covet.  

However, I argue that the "Ten Commandments" is not a standalone package, but it's part of a larger covenant (in fact, if you want to discuss second millennium B.C. suzerain-vassal treaties sometimes, I'm game!). A covenant is an agreement made between two parties, and in this context (as with suzerain-vassal), it is offered by a greater power to a lesser power: like Assyria to Judah, Rome to Herod, or, in this obvious case, the LORD to the children of Israel.  

All these treaties began with historical context. But while most started with a beatdown: "Whereas we're big, you're small, we thwomped you, our gods are better...", this one starts with redemption: "I am the LORD, who rescued you out of slavery."  Not a defeat, but a rescue.  

So note this, for it's critical to understanding God: He acted first, and acted lovingly for the good of Israel, before He ever placed expectations on them. This theme is consistent throughout Scripture, but explicitly explained here.  

Whenever someone criticizes God for having expectations of us, remember: as in Egypt, as with the cross, He made the first move, and it was for our benefit.