"上了" is to express you are getting to love/like sth,it is like" I tend to do sth." such as:我喜欢上了爵士means I don't like Jazz before but now I am getting used to loving it.
相爱:you can see 相互 is "each other", in this sentence, they love each other, so we use 相爱means I love you and you also love me not just like the first sentence only one part.
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