What are some reasons that the doctor might tell me to have a splenectomy?

Cause/symptoms. It depends on why it is enlarged and if it is causing symptoms. The mere presence of an enlarged spleen does not necessarily mean an operation is required. A discussion on the pros and cons with your surgeon is advised.
Tumors or bleeding. Most of the times spleens are removed its for large tumors or bleeding issues from low platelets or direct trauma and bleeding from the spleen itself.