Objection 1. It would seem that ecstasy is not an effect of love. For ecstasy seems to imply loss of reason. But love does not always result in loss of reason: for lovers are masters of themselves at times. Therefore love does not cause ecstasy.
Objection 2. Further, the lover desires the beloved to be united to him. Therefore he draws the beloved to himself, rather than betakes himself into the beloved, going forth out from himself as it were.
Objection 3. Further, love unites the beloved to the lover, as stated above (Article 1). If, therefore, the lover goes out from himself, in order to betake himself into the beloved, it follows that the lover always loves the beloved more than himself: which is evidently false. Therefore ecstasy is not an effect of love.
On the contrary, Dionysius says (Div. Nom. iv) that "the Divine love produces ecstasy," and that "God Himself suffered ecstasy through love." Since therefore according to the same author (Div. Nom. iv), every love is a participated likeness of the Divine Love, it seems that every love causes ecstasy.
I answer that, To suffer ecstasy means to be placed outside oneself. This happens as to the apprehensive power and as to the appetitive power. As to the apprehensive power, a man is said to be placed outside himself, when he is placed outside the knowledge proper to him. This may be due to his being raised to a higher knowledge; thus, a man is said to suffer ecstasy, inasmuch as he is placed outside the connatural apprehension of his sense and reason, when he is raised up so as to comprehend things that surpass sense and reason: or it may be due to his being cast down into a state of debasement; thus a man may be said to suffer ecstasy, when he is overcome by violent passion or madness. As to the appetitive power, a man is said to suffer ecstasy, when that power is borne towards something else, so that it goes forth out from itself, as it were.
The first of these ecstasies is caused by love dispositively in so far, namely, as love makes the lover dwell on the beloved, as stated above (Article 2), and to dwell intently on one thing draws the mind from other things. The second ecstasy is caused by love directly; by love of friendship, simply; by love of concupiscence not simply but in a restricted sense. Because in love of concupiscence, the lover is carried out of himself, in a certain sense; in so far, namely, as not being satisfied with enjoying the good that he has, he seeks to enjoy something outside himself. But since he seeks to have this extrinsic good for himself, he does not go out from himself simply, and this movement remains finally within him. On the other hand, in the love of friendship, a man's affection goes out from itself simply; because he wishes and does good to his friend, by caring and providing for him, for his sake.
Reply to Objection 1. This argument is true of the first kind of ecstasy.
Reply to Objection 2. This argument applies to love of concupiscence, which, as stated above, does not cause ecstasy simply.
Reply to Objection 3. He who loves, goes out from himself, in so far as he wills the good of his friend and works for it. Yet he does not will the good of his friend more than his own good: and so it does not follow that he loves another more than himself.
Source: Summa Theologica (New Advent)