Dont know if someone thought this already, but I think the big raaz is this that gauri's grandfather must have put the property which devika has, in the name of this first grandchild which is gauri.
When Mahen came to know of this after shashank's engagement to gauri, he took advantage of the various situations and told shashank to get out of her life.
It would be very beneficial for him becoz if devika is his rival then they have equal amout of property. And if this is in gauri's name, Gauri being gauri would have handed the property to Mahen in a platter.
Let me know if its been already thought