Objection 1. It would seem that hope is not a help but a hindrance to action. Because hope implies security. But security begets negligence which hinders action. Therefore hope is a hindrance to action.
Objection 2. Further, sorrow hinders action, as stated above (Question 37, Article 3). But hope sometimes causes sorrow: for it is written (Proverbs 13:12): "Hope that is deferred afflicteth the soul." Therefore hope hinders action.
Objection 3. Further, despair is contrary to hope, as stated above (Article 4). But despair, especially in matters of war, conduces to action; for it is written (2 Samuel 2:26), that "it is dangerous to drive people to despair." Therefore hope has a contrary effect, namely, by hindering action.
On the contrary, It is written (1 Corinthians 9:10) that "he that plougheth should plough in hope... to receive fruit": and the same applies to all other actions.
I answer that, Hope of its very nature is a help to action by making it more intense: and this for two reasons. First, by reason of its object, which is a good, difficult but possible. For the thought of its being difficult arouses our attention; while the thought that it is possible is no drag on our effort. Hence it follows that by reason of hope man is intent on his action. Secondly, on account of its effect. Because hope, as stated above (Question 32, Article 3), causes pleasure; which is a help to action, as stated above (Question 33, Article 4). Therefore hope is conducive to action.
Reply to Objection 1. Hope regards a good to be obtained; security regards an evil to be avoided. Wherefore security seems to be contrary to fear rather than to belong to hope. Yet security does not beget negligence, save in so far as it lessens the idea of difficulty: whereby it also lessens the character of hope: for the things in which a man fears no hindrance, are no longer looked upon as difficult.
Reply to Objection 2. Hope of itself causes pleasure; it is by accident that it causes sorrow, as stated above (32, 3, ad 2).
Reply to Objection 3. Despair threatens danger in war, on account of a certain hope that attaches to it. For they who despair of flight, strive less to fly, but hope to avenge their death: and therefore in this hope they fight the more bravely, and consequently prove dangerous to the foe.
Source: Summa Theologica (New Advent)