Question

Why Lenz's Law? Why must the solenoid make itself as a pole to oppose the pole of the bar magnet going into or out from it?
For example, when a bar magnet approaches a solenoid (that is not connected to any power source), a repulsive force is produced. The repulsive force is caused by the current flow in the solenoid that creates a like pole to repel the bar magnet. Why then? Why can't it just attract it? I found on some websites that it's related to work done and the conservation of momentum. Need some light please.