Objection 1. It seems that fear decreases when charity increases. For Augustine says (In prim. canon. Joan. Tract. ix): "The more charity increases, the more fear decreases."
Objection 2. Further, fear decreases when hope increases. But charity increases when hope increases, as stated above (Question 17, Article 8). Therefore fear decreases when charity increases.
Objection 3. Further, love implies union, whereas fear implies separation. Now separation decreases when union increases. Therefore fear decreases when the love of charity increases.
On the contrary, Augustine says (Qq. lxxxiii, qu. 36) that "the fear of God not only begins but also perfects wisdom, whereby we love God above all things, and our neighbor as ourselves."
I answer that, Fear is twofold, as stated above (A2,4); one is filial fear, whereby a son fears to offend his father or to be separated from him; the other is servile fear, whereby one fears punishment.
Now filial fear must needs increase when charity increases, even as an effect increases with the increase of its cause. For the more one loves a man, the more one fears to offend him and to be separated from him.
On the other hand servile fear, as regards its servility, is entirely cast out when charity comes, although the fear of punishment remains as to its substance, as stated above (Article 6). This fear decreases as charity increases, chiefly as regards its act, since the more a man loves God, the less he fears punishment; first, because he thinks less of his own good, to which punishment is opposed; secondly, because, the faster he clings, the more confident he is of the reward, and, consequently the less fearful of punishment.
Reply to Objection 1. Augustine speaks there of the fear of punishment.
Reply to Objection 2. It is fear of punishment that decreases when hope increases; but with the increase of the latter filial fear increases, because the more certainly a man expects to obtain a good by another's help, the more he fears to offend him or to be separated from him.
Reply to Objection 3. Filial fear does not imply separation from God, but submission to Him, and shuns separation from that submission. Yet, in a way, it implies separation, in the point of not presuming to equal oneself to Him, and of submitting to Him, which separation is to be observed even in charity, in so far as a man loves God more than himself and more than aught else. Hence the increase of the love of charity implies not a decrease but an increase in the reverence of fear.
Source: Summa Theologica (New Advent)