In the Name of Allah, The Most Gracious, Most Merciful
Why does the one who fasts have two joys?
If someone refrains from Siyaam, he will feel remoteness from Allah the Almighty and will look at himself as a sinner.
When Eid comes and people celebrate it and break their Siyaam, the one who does not observe Siyaam will not have the same feeling of those who observe Siyaam during Ramadan. Well, he will just eat the same way he uses to do during Ramadan.
Ramadan is a temporary test, so the one who fasts will be joyful (in Eid), because he will have passed the test, and he will have felt that he observes Siyaam out of faith and seeking Allah's Rewards, that he obeys Allah and submits to His Way and that the distress of Siyaam is over.