Joshua 21:45 Not one word of all the good promises that the Lord had made to the house of Israel had failed; all came to pass.
Simply put - God keeps His promises. Always.
Unfortunately, He's also the only One that can be counted on to always keep all of His promises. We don't. We should, and we try, but inevitably we fail someone - sometimes it's within our control, and sometimes it's not in our control - but the result is the same nonetheless.
We cannot put our trust 100% in anyone other than God. Yes, your spouse should be faithful. Yes, your best friend should be trustworthy. Yes, your sister and your brother should keep their word. But the reality is, that we are all human, and we will all fail those we love at some point.
Obviously, this is not an excuse for us not to attempt to be faithful! In fact, it's a good reminder to us to be more purposeful about keeping our promises to our loved ones.
But above all, it's a reassurance that though others may fail you, God never will.