Does a landlord have to give a reason if they refuse to consent to my lease assignment?

It depends on your lease's specific wording, since there is no general Ontario rule forcing a landlord to justify a refusal outside of what the lease itself requires. Where your lease includes a standard that consent cannot be unreasonably withheld, a landlord who refuses without giving any reason at all puts themselves in a weak position — if the refusal is ever challenged, an unexplained "no" is very hard to defend as reasonable, since there's nothing on the record to show it was based on a legitimate concern.

In practice, this means a landlord who wants a refusal to hold up is generally well advised to state real, specific grounds — financial concerns about the buyer, a change in permitted use, or similar — and a tenant facing silence or a bare refusal has a reasonable basis to push for those reasons in writing before assuming the refusal is final.

If your landlord won't explain a refusal, ask for the grounds in writing and don't treat silence as the end of the conversation. A Treadstone business lawyer can help press for a proper explanation and assess whether it holds up.