王菲－你快乐所以我快乐 Faye Wong- You're happy, so I'm happy

My favorite Faye Wong song.
我最爱王菲的作品

Her usage of the conjunctions "所以” and “于是” confuses me. We were taught by rote that their usage differ in that two clauses conjoined using 所以 form a causal conditional, where the event described in the second sentence is causally dependent on that of the first. 于是 on the other hand is found describing related, but not necessarily dependent events in a sequence. The singer uses both seemingly interchangeably. This bothers me.

她说“所以”和“于是”这两个连词的用法让我很怀疑。 我们在教室里时候被教这了这些东西的用法并不是一样的。