Good question. Interestingly, I just came across the same idea in another lesson: Cuando nos casemos, seremos felicísimas las dos.
My Grammar book [by Butt & Benjamin] implies that the customary use [in Spain] of 'ser' before 'feliz' can be regarded as an 'anomaly', i.e., it does not follow the logical rule [which says that 'estar' should be used if it describes a 'temporary or transitory state' or a 'condition']. Thus, there is little or no difference between "Estoy contento" and "Soy feliz"... The above book also mentions that [in Spain] you will usually see "ser pobre" even when that is referring to a temporary state... [in Hispano-America, on the other hand, you will often hear 'estar pobre' and 'estar feliz']
The general rule says to use estar for feelings, but for some reason (not a logical one I'm afraid) in Spain we have a tendency to use the adjective "feliz" with ser more than estar, even when the situation is understood as a temporary state of mind.
What you cannot do is use "ser" with "contento". You always need estar with contento.
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