In the reading for today's Office, St. Therese of Lisieux explains how she came to understand her vocation. She was reading St. Paul's letter to the Corinthians on the different gifts in the Church. Therese didn't see herself in any of the gifts Paul mentions (preacher, teacher, apostle, etc.). So she felt unsatisfied, but then she came to Paul's beautiful hymn on love (ch. 13). And in a sudden burst of light, she knew her vocation: "The Church has a heart...and in the heart of the Church I will be love."
The other day I came across something in St. Thomas that relates to this. In speaking of how the Trinity dwells in us through grace, he says "the Son is the Word, but not just any word, the Word breathing love."* Thomas sometimes lets some poetry slip into his theology, and this is a beautiful example of it. Through grace, the Father, Son, and Holy Spirit dwell in our souls, and the Son is "the Word breathing love."
St Therese may never have read that line from St. Thomas. But she lived it to the fullest degree--and that's why she's a Doctor of the Church. She allowed the Trinity to dwell more and more in her soul, to the point where she lived and breathed love. If we are in the state of grace, the Trinity dwells in us too. The Son lives in us, breathing forth the Holy Spirit, who is Love. As we go about our daily tasks, we too can live and breathe love to all the people around us, just like Therese did.
* (I, q. 43, a. 5, ad 2--Blackfriars translation).