I guess it deals with category of definiteness/indefiniteness in Russian. Definite adjective –that that refers to the certain noun or to certain circumstances.

Я был счастлив/счастливый -I was happy always, happiness was my pertinent quality. (indefinite adjective)
Я был счастливым -I was happy at that moment. (definite adjective)

By the way, the same applies to nouns also.
Cf:
He was captain – он был капитан .
He was the captain [of the ship] - он был капитаном [корабля].

It is not a rule in modern Russian, but historically ,I think, it was so.