Question

Reasons of conflict in India and Pakistan?

Answer 1

There are many reasons for discord between India and Pakistan like.
Cross-border terrorism
Kashmir Issue
Indus water treaty
Troubled past and separation of subcontinent

Explanation:

I will assume that you are asking a question about reasons of conflict between India and Pakistan and not the reasons of conflict in India & Pakistan.

Writing a geopolitically neutral answer to this question is very difficult, but I will try my best. The aforementioned points are only a tip of an iceberg and there many more issues which I have not mentioned.

The division of subcontinent based on different religious ideology in 1947 opened a Pandora's box. The mass movement of humans from one side to another led to riots in which millions tragically died.
In subsequent years there were numerous border conflicts between the two nations which led to Ind and Pakistan fighting wars in 1965, 1971 and 1999.

The war of 1971 led to the formation of the sovereign state of Bangladesh from erstwhile East Pakistan.

In recent years the conflict on subcontinent has taken a turn for the worst as both the conflicting parties now possess Nuclear weapons which further increases the tension on the Indian subcontinent.

It is only a brief and rather insufficient analysis of Ind-Pak relations.

I would suggest you read Khuswant Singh's "A train to Pakistan", the book tells the story of the partition of Indian Subcontinent.