Possibly he knew his end was close, and this painting seems to burst with a manic energy, as though he were desperate to get this deeply personal piece down onto canvas whilst there was still time.
The painting is clearly close to El Greco's heart. He intended a version of this to hang in his family tomb and it sings with the fervour of his Catholic beliefs. Certainly religious intensity is a theme El Greco returned to again and again throughout his career in such paintings as The Holy Trinity and The Disrobing of Christ; and his faith informed his art to such an extent that his departure from Italy derived in part from his considering Michelangelo's Sistine Chapel paintings to not be Christian enough. El Greco could be said to be the last truly great religious painter, in terms of one whose art was informed by their faith, and this is a truly great religious painting.
Towards the end of his career, El Greco's use of shape and height became more elastic and free int their usage, in the fashion of such Mannerist painters as Tintoretto. In Adoration of the Shepherds this elongation, this playing with height, reaches new and dizzying extremes. The Shepherds' impossibly elongated forms seem almost to be reaching to the angels above. the unnatural twisting and contortion of the bodies is typical of El Greco, and in part led to him perhaps being undervalued in his time by contemporary critics of the Spanish Renaissance. But this playing with form is revolutionary, whilst laying the groundwork for Modernism, 400 years later, it simultaneously has a foot in the past, in the Byzantine tradition of icon painting.
The Adoration of the Shepherds is beautifully composed. With the figures circulating around a luminous, glowing Christ child. The light in the painting seems to come from Christ himself, and this otherworldly light contriubutes to the extraordinary use of colour. El Greco was bolder than any of his contemporaries (and, arguably, anyone until Delacroix in the 19th century) in his use of pigmentation, and in Adoration of the Shepherds intense and vivid reds and blues provide a dramatic contrast to the darkness around the edge of the painting. It implies the light of Divinity, juxtaposing it with the darkness of the world away from Christ. This is a poem which says that the Divine is with us every day.
As the Shepherds adore, angels circulate above. Again, El Greco has played with traditional notions of composition, the angels bodies are foreshortened, their frames at curious angles. The effect of this, mirroring the circling of the adoring Shepherds, is to imbue the painting with a great deal of eneergy, there's a very real sense of motion, of these people, and by extension humanity, revolving around this peaceful, glowing Christ at the centre of all things.
The Adoration of the Shepherds is in many ways the culmination of the elements which made El Greco the artist he was. It combines his religiosty, the strongest recurring theme in his work with possibly the strongest expression of his contortion of form. It could be said to be the apogee of his work; it draws together many traditions, standing at the crux of Mannerism and the Baroque, marrying Western and Eastern traditions in one dynamic, striking canvas.