WB Yeats, "the Master of Symbolism" acquired his taste from the French literati, who taught him how the use of melodic, rythmic, and emotive words can convey a spiritual meaning upon the reader.
To Yeats, the symbol is the missing link between the metaphor and the romantic ideal. The use of symbols is a higher literary strategy which moves the imagination, as well as the heart and the spirit. He is quoted as saying: 'metaphors are not profound enough to be moving,' which is his reason behind providing rich, vivid, nostalgic, and poignant symbology in his works.
In his Symbolism and Poetry, he conveys the need that these symbols are direct, short, to the point, and rich enough to inspire. He understands that the use of well-selected symbols entice and enrich the entire literary experience.
Yeats’s early poetry, his first volumes in particular, were influenced by the symbolism of William Blake and Irish folklore and myth. This poetry is more romantic and dreamlike than his later work, which is generally more highly regarded.
Composed in 1900, Yeats's influential essay "The Symbolism of Poetry" offers an extended definition of symbolism and a meditation on the nature of poetry in general. I checked and there are many full text copies available, one on answers.com.I think it would be really important to have a look at this to gain insight into what the poet himself thought about Symbolism and its place in his own oeuvre. Another work worth a look is Arthur Symons, The Symbolist movement in literature, which is dedicated to William Butler Yeats and examines the poetic output of Yeats (and others) in terms of how it fits into the Symbolist Movement.