Matthew 5:23-24 KJV
23 Therefore if thou bring thy gift to the altar, and there rememberest that thy brother hath ought against thee; 24 Leave there thy gift before the altar, and go thy way; first be reconciled to thy brother, and then come and offer thy gift.
It doesn't take long of studying Jesus' life to learn that relationships are very important. These verses are no different. Here we see that it is more important to keep relationship than it is to offer a sacrifice. Also, can you truly offer a sacrifice if your heart isn't at peace?
What I find interesting (and never noticed before) is that it says, if your brother "hath ought against thee." This isn't whether you have ill feelings against someone else, but how he feels about you. As Christians we should strive to live peaceably with others ("If it be possible, as much as lieth in you, live peaceably with all men." ~Romans 12:18). We need to do whatever we can to have and keep that peace.