1.Limited water and too many users in southern India. 2.Lots and lots of water below the himalayas gets flooded and wasted towards the ocean. 3.Tamil nadu lacks water resources in its territory and depends on other states. 4.If we go by the river origin logic Cauvery originally belongs to the people of coorg(Kodagu).I did see some news articles that Kodagu wants a seperate statehood. If this state hood is achieved then Karnataka faces a similar situation as faced by TN. 5. States at a higher altitude are always at advantage and can bully the lower altitude regions.To control this scenario tribunal was setup.
If Karnataka does not accept the tribunal award then it is a ever green problem and a never ending story.There cannot be solution for this.The current central government is dominated by ministers from Tamil Nadu.