The CCC states:
2147 Promises made to others in God’s name engage the divine honor, fidelity, truthfulness, and authority. They must be respected in justice. To be unfaithful to them is to misuse God’s name and in some way to make God out to be a liar.77
2153 In the Sermon on the Mount, Jesus explained the second commandment: "You have heard that it was said to the men of old, ‘You shall not swear falsely, but shall perform to the Lord what you have sworn.’ But I say to you, Do not swear at all. . . . Let what you say be simply ‘Yes’ or ‘No’; anything more than this comes from the evil one."82 Jesus teaches that every oath involves a reference to God and that God’s presence and his truth must be honored in all speech. Discretion in calling upon God is allied with a respectful awareness of his presence, which all our assertions either witness to or mock.
2154 Following St. Paul,83 the tradition of the Church has understood Jesus’ words as not excluding oaths made for grave and right reasons (for example, in court). "An oath, that is the invocation of the divine name as a witness to truth, cannot be taken unless in truth, in judgment, and in justice."84
2155 The holiness of the divine name demands that we neither use it for trivial matters, nor take an oath which on the basis of the circumstances could be interpreted as approval of an authority unjustly requiring it. When an oath is required by illegitimate civil authorities, it may be refused. It must be refused when it is required for purposes contrary to the dignity of persons or to ecclesial communion.
It seems to me it’s about integrity. If I am a person of my word, then I really don’t have a need to make promises and oaths to assure someone I will do what I say I will. That’s what we should strive for–to be honest and to keep our word. We shouldn’t agree to do something, and then blow it off or make excuses or forget–it should be important enough to us to keep our word, so that we complete the tasks we’ve agreed to do. We don’t want to be liars. If we aren’t fairly certain we can and will do it, we shouldn’t be saying we will in the first place.
Just my thoughts.