Why is it necessary to share power among different social groups in a democracy?

It is necessary to share power among different social groups in a democracy because of moral and prudential reasons:

• Prudentially because power sharing helps to reduce the possibility of conflict between social groups. Since social conflict often leads to violence and political instability, power sharing is a good way to ensure the stability of political order. Imposing the will of majority community over others may look like an attractive option in the short run, but in the long run it undermines the unity of the nation. Tyranny of the majority is not just oppressive for the minority; it often brings ruin to the majority as well. 
•  Morally or deeper reason is that sharing is the very spirit of democracy. A democratic rule involves sharing power with those affected by its exercise, and who have to live with its effects. People have a right to be consulted on how they are to be governed. A legitimate government is one where citizens, through participation, acquire a stake in the system.