The first thing that we see is the people's reaction to it. They were disappointed that their hopes of fresh drinking water was gone. The bitterness caused them to complain about their disappointment. In interesting fact here is that the bitterness caused them to go from disappointment to complaining, a downward spiral. Bitterness doesn't help us to look up to God for help, instead it is like a whirlpool that sucks us down and away from God. The Children of Israel three days prior had walked on dry ground and were praising God for what He did in parting the Red Sea. Now they were complaining that He had led them there to die from the lack of water. Bitterness causes us to forget what God has done for us because we are looking at the here and now as to how we feel.
Secondly, bitterness makes us unusable. The people were disappointed because they couldn't use the water as they had hoped for. When we are bitter God cannot use us. Think about it. How could God use someone that is complaining about what is going on in their lives? As long as I was holding on to the bitterness in my life I was unwilling to let God use me the way wanted me to be used. When He had asked me to start writing 10 years ago I struggled and fought with Him about it. I won't say that I'm 100% bitterness free but things can happen that would set off the hurts no longer set me off. Now I look forward to doing what God has asked me to do.
Thirdly, bitterness stops growth. Nothing grows around salt water. It kills grass, weeds, snails, slugs.... Likewise bitterness prevents us from growing in our relationships with others. When we have used it on a hurt it is there preventing us from getting close to someone else because they might hurt us in the same way so it walls us off from getting close to others and the relation can't grow because bitterness keeps us from getting close. Bitterness kills the joy in our lives. No matter how hard joy tries to grow bitterness is there to choke it out. Most importantly, bitterness stops our growth as a Christian. Many times the bitterness causes us to blame God for allowing the hurt to happen. Like the Children of Israel blamed God for the bitter water and leading them to their death. Bitterness blinds us from what God has done, is doing and will do in our lives.
Bitterness like most things that we do on our own power has the letter "I" in it. It may not be in the center as it is in prIde and sIn. It is me take God's job as the healer of my life. It says that God is not good enough to heal this hurt and I know better how to take dare of my wounds. When my parents went through a divorce my mom had a 3x5 card on the refrigerator that said, "Lord make me better not bitter." May that be our saying too.