Hi there! I've answered a similar question to this before, when I discussed why Toronto surpassed Montreal as Canada's largest city. I don't know that I necessarily have anything to add to what I've said there, but I will briefly summarize.

Toronto is situated in the Great Lakes, and as such is close to many of the largest and most industrialized cities in America such as Chicago and Detroit. It also connected to the Atlantic via the St. Lawrence River, which flows northeast from Lake Ontario and the other Great Lakes to the ocean.

Another key development was the completion of the St. Lawrence Seaway project in 1959, a major industrial and engineering undertaking that opened up the river to commercial shipping all the way from Montreal to the entire Great Lakes basin. What this meant was that transatlantic commercial ships which previously would have unloaded in Montreal could now unload in Toronto, much closer to the industrial heartland of both Canada and the United States. As a result, Toronto is not as isolated or as inland as you might imagine - no more, say, than London in the UK.

These geographic advantages are combined with a number of other attractive features. Toronto is the financial, cultural, and industrial centre of English-speaking Canada. All of the major banks in Canada have their headquarters in Toronto, it is a major hub for film and television, and has first-rate universities like York University and the University of Toronto.

These factors have made the city a destination not only for internal migrants within Canada but also for immigrants from abroad, who flock to the city for its multicultural character and job opportunities.

I recommended some sources in my previous answer, and I especially suggest reading Permeable Border: The Great Lakes Region as Transnational Region, 1650-1990 (2005) if you have access to a university library since it offers an interesting breakdown on the historical development of the Great Lakes megalopolis.