One of the attributes of G-d is that He says and delivers, as we say in our daily prayer of Barukh She-amar: "Blessed be He who says and does". Since we are created in the image of G-d, it behooves us to follow in G-d's footsteps. When we say that we will do something, we must deliver. When we promise, we must fulfill those promises.
We all appreciate and value people whose words are like gold. Such people, though scarce, are reliable, and infuse in us a sense of confidence and tranquility. Our holy Torah stresses the importance of this quality: "... he shall not desecrate his word, whatever comes from his mouth he shall do" (Bamidbar 30:3). When we are particular about the value of our promises, G-d in return, acts with us the same way. G-d will ensure that our words will get carried out.
There is a story about Ribbi Aqiba, who borrowed money from a Roman matron. The Roman matron told Ribbi Aqiba that G-d and the ocean will be their witnesses. Ribbi Aqiba fell ill on the day that his debt was due and could not return it.
The Roman matron went to the ocean and said: "G-d, it is known before You, that today is the day that Ribbi Aqiba agreed to return the money he had borrowed from me, but hasn't come. Since You and the ocean are the guarantors, I ask of You to return my money!". How did He react? G-d caused one princess whose palace was at the sea shore to lose her mind and throw a box full of precious stones into the ocean. The waves "brought" the box to the matron, who exchanged them for the money and held on to the balance.
When Ribbi Aqiba recovered, he came to the matron to repay his debt. She, in return, gave him the balance from the treasure box. The revered Hakham, the Ben Ish Hai, explains that G-d could have returned the money to the Roman matron in many miraculous ways. However, since Ribbi Aqiba was extremely careful with his words and promises, G-d made this miracle happen specifically through the ocean, which was chosen as a guarantor by Ribbi Aqiba.