Frank, 

have a look at the lesson linked below. 

The neuter object pronoun ‘ le ‘ is used to avoid repetition of the adjective ‘ heureux ‘ as it cannot simply be left out of the 2nd verbal clause as is done in English. 

Without using ‘ le’ the phrase would be ‘ … pour que tu sois aussi heureuse que je suis heureux aujourd’hui. ‘ 

Just saying ‘ .. que je suis aujourd’hui ‘ is incomplete in French.

‘Using neuter pronoun le to replace adjectives (French Direct Object Pronouns)