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Why doesn't aluminium fuse directly to copper? What determines the kinds of metals that can be fused or amalgamated among each other?

Hey there kind scholars! 

My girlfriend and I have a strange habit of watching old episodes of that Discovery channel show, "How it's made", to relax and sleep. Last night we watched one episode on "Thermal machines" (such as air conditioning units and refrigerators). On a certain moment during the fabrication of the air conditioner, the narrator said that a certain piece of the aluminium "net" component had to be welded to copper using a zinc-aluminium alloy. 

The question(s) then is (are):

Why can't the copper and aluminium be welded directly? 

What characteristic on a metal determines it to be able to be welded or amalgamated to another?

Edit 1: a comma.
gpt
If you weld copper and aluminum directly, the resulting alloy in the middle is likely to form brittle Al-Cu compounds. Also, welding copper is expensive. It needs a lot of power because it's such a good heat sink and you generally need to use expensive helium as a shielding gas. 

Aluminum-zinc is a brazing alloy. It allows you to join them without having to melt the base metal. It melts at a lower temperature than copper and conventional Al alloys. It dissolves the surface of the surface of the base metals to bond with it, but if done correctly, not enough to form any undesirable compounds. This is essentially the same thing as soldering, just at a higher temperature.