What would stop an LSR Router in an MPLS environment from forming neighborship with another LSR router? Kindly elaborate.

there are a lot of factors that can prevent2 LSR routers to form a neighbor relationship

ACLs that filter ldp or tdp traffic or authentication misconfigured   or one side runs ldp and the other side running tdp , routing could be an issue if the router uses the loopback interface as the source and the other side doesn't have that prefix in the routing table