Read Psalms 66:18-20  Verse 18 is a conditional verse.  That means that if we do something, then God will do something.  It also means that if we do not do something, then God will not do something.  We would like to know that God will hear our prayer.  But verse 18 says if we are disobedient and have sin in our lives, then God will not listen to our prayers.  This does not mean that God cannot hear our prayers; it just means that God will not listen to our prayers because we are not listening to God.  But then verse 19 says that God will hear our prayer because we obey Him.  Finally in verse 20, David blesses the Lord for listening to and answering his prayer.  We should obey God because we love Him.  But we should also obey God because if we don't, when we need Him to hear our prayer, He will not listen to us.  It is much better to obey God and to know that He is listening to us.  Isn't it good to know that God loves us?  We need Him to hear our prayers.