Maybe their definition of happy is different.... if they're both work driven individuals maybe they're together because it works....
In life we have our loves... our vices. Some want money, fame, success, love, family, friendship and there are so many levels to each. Subconsciously we list all of our items from most important to least important.... then we start looking for a mate. You start with an attraction... whether it be physical, psychological, spiritual.... and when you meet someone you catagorize them... if you're goal in life is to be a married woman with 4 children then you cannot be with a person who's goal is to a successful CEO who travels and doesn't want children.
If you are a strong willed, independant person with particular goals... you find someone who has that same drive, that same lifestyle... you pick within the offerings that you're given. Maybe He was the best she could get within her "choosings"... and vise versa... maybe they're not an ideal couple... they just choose to be because their number one wasn't love... it was something with a bit of self preservation... a bit of selfishness you could call it.