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1. Because of faster oxidization due to the contact with copper.
2. Due to different electrical conductivities, these two materials have different heating properties and thus with repeated heating and cooling the connection is weakened. Then it heats even more due to less connection.
3. With minimal humidity there is chemical electrolysis, which leads to corrosion, which leads to weaker connection, which leads to all other fun stuff, like fun sparkles.
So you definitely want aluminium + copper.