This is what the Lord says:

“What fault did your ancestors find in me,
    that they strayed so far from me?
They followed worthless idols
    and became worthless themselves. Jeremiah 2:5

In the book of Jeremiah, God wonders why the people don’t even question the broken relationship between them and God, instead wondering where God is in their lives.  The people stray so casually and naturally they don’t even know they’re off course. 

God is surprised because He’s done so much for us.  For the Israelites, He saved them from Egypt, protected them from enemies (many times miraculously) and still they forgot about God. 

Even more surprisingly, the priests and the prophets completely forgot about Him (2:8,9).  That’s got to befuddle God.

God knows why.  The people create their own forms of spiritual replenishment (2:13) but those dry up.  They build their own gods of wood and stone and call it their god and father (2:27).  God called them wild vines growing wherever they wanted or donkeys in heat sniffing out the next relationship (2:24).  The people cannot cleanse themselves of their sins (2:22).  And yet the people blame God (2:29).

But if God knows why and knew they would reject Him, is He “surprised” by people’s rejection.  He’s not surprised in the sense that He didn’t see it coming, but He is surprised because there’s so much evidence and benefit in having a relationship with Him that He can’t believe people can be so stubborn and reject him.