Objection 1. It would seem that hope is not a cause of love. Because, according to Augustine (De Civ. Dei xiv, 7,9), love is the first of the soul's emotions. But hope is an emotion of the soul. Therefore love precedes hope, and consequently hope does not cause love.
Objection 2. Further, desire precedes hope. But desire is caused by love, as stated above (Question 25, Article 2). Therefore hope, too, follows love, and consequently is not its cause.
Objection 3. Further, hope causes pleasure, as stated above (Question 32, Article 3). But pleasure is only of the good that is loved. Therefore love precedes hope.
On the contrary, The gloss commenting on Matthew 1:2, "Abraham begot Isaac, and Isaac begot Jacob," says, i.e. "faith begets hope, and hope begets charity." But charity is love. Therefore love is caused by hope.
I answer that, Hope can regard two things. For it regards as its object, the good which one hopes for. But since the good we hope for is something difficult but possible to obtain; and since it happens sometimes that what is difficult becomes possible to us, not through ourselves but through others; hence it is that hope regards also that by which something becomes possible to us.
In so far, then, as hope regards the good we hope to get, it is caused by love: since we do not hope save for that which we desire and love. But in so far as hope regards one through whom something becomes possible to us, love is caused by hope, and not vice versa. Because by the very fact that we hope that good will accrue to us through someone, we are moved towards him as to our own good; and thus we begin to love him. Whereas from the fact that we love someone we do not hope in him, except accidentally, that is, in so far as we think that he returns our love. Wherefore the fact of being loved by another makes us hope in him; but our love for him is caused by the hope we have in him.
Wherefore the Replies to the Objections are evident.
Source: Summa Theologica (New Advent)