Of the 5 Lucy poems this one is the prime example of the Wordsworthian endeavor to trace the associations we make in a state of excitement.
Doubtless, there has been a time when you were passionately in love with someone and they you. In the height of your love, while you made your way to your lovers house, you had a moment's terror that they wouldn't be there or that they would leave you. Maybe, they have lost interest in you or found someone else to love. Or possibly, they inexplicably die.
This is the experience of Wordsworth's strange fit of passion. The aim of the poem is to explore the origins of the associations which led to this strange conclusion: that his lover Lucy would be dead.