Why do 2 solenoids attract each other?

You can approximate a solenoid as a series of current loops. When we look at two loops of opposing current it looks as follows

The black lines are current loops. Red indicates magnetic field and blue indicates force. We determine the effect of the left loop's magnetic field on the right loop. Using the right hand rule$^dagger$ we can get the direction of the magnetic field (little red circle). This gives us the direction of the magnetic field on the right loop. Using another right hand rule we can determine the direction of force. On the right hand side of the right loop we have the same force but in the opposite direction. Because it's farther away the force is weaker and the net force is attractive. We would have to sum over all the little forces in the loop to get an exact answer.

It sounds like you were imagining both loops of current to be rotating in the same way so both loops would be copies of each other. This can't be right: if you were to replace two identical loops with bar magnets they would be pointing in the same direction. Since north-north and south-south matches up they would be repelling.

$dagger$ What is this black magic called a left hand rule??