The two main ways Larkin creates a feeling of excitement or anticipation is through the poem's title and through the repeated line that comes halfway through the verses.
The title "Coming" suggests that something is about to arrive, which raises our curiosity and excitement. The use of the participle ("ing" ending) provides a feeling of motion. Whatever is going to arrive is already on the way.
This sense of anticipation continues as the thrush sings, "astonishing" the brickwork. The word "astonishing" is startling in the midst of quiet lines about light bathing the "serene / Foreheads" of houses. The movement from houses described as serene to houses described as astonished conveys a rising sense of excitement. Something surprising seems to be on the way.
The pay off comes in the middle of the poem, in the lines:
It will be spring soon,
It will be spring soon --
The repeated singsong cadences of these two lines express a childlike excitement at the changing of the seasons from darker, quieter winter to the sunshine and birds' singing of spring. The words are simple, and one can imagine a child bouncing or jumping up and down with excitement at this realization that warm weather is on the way. The repetition emphasizes the importance of these lines.
The dash is also important, for after this rise in excitement at the arrival of spring, the poem turns to a more mature, adult reflection that nevertheless ends on the childlike word "happy."