Answer by Siddharth Jayshankar:
A political party is formed with the motive of capturing power and using it as an instrument to serve the people in the method they believe is correct. The leader of the party spreads its ideology among the masses and several people become its members. That is how the party grows. Most people will join a party only if it can come to power or has a reasonable chance of doing so. The members of the party are driven by a desire to come to power and retain it.
If a party repeatedly loses elections and remains out of power, it will face an existential crisis. The party workers and members will get demoralised and lose the will to campaign. They might even quit the party and go to another party. It will lose all organisational support. Its vote share will keep declining, the party will lose its influence and ultimately it might go extinct. Find out what happened to congress in Tamil Nadu, West Bengal, Gujarat.
For a party in power, it is quite easy to spread its support base. It has the entire state machinery at its command. Several businessmen and industrialist donate to the party to keep close ties with it. This keeps the party financially afloat. The party can draft policies for the people and familiarise itself with them. It can also appoint its supporters to key positions.
In the case of the Shiv Sena, the party has been out of power for 15 years. In the recently concluded elections, its alliance with the BJP broke. The Shiv Sena won only 63 seats and came second. If the BJP goes ahead and forms a minority government, the Sena will not have any control over the state machinery. The BJP, which is already upbeat can use its control over the state government to grow itself in the state. (It has a huge potential to do so). It can launch several policies and increase its goodwill among the people.
The Shiv Sena is worried because of this. Several Shiv Sena members might defect to the BJP weakening the Sena. The BJP can also withdraw support to the Sena in the BMC and cut its main source of income. All this will put the Sena into a very tough spot. That is why it wants to be in power.
But it is hesitant to join the BJP. Read the reasons for that over here: