Trusting in the Lord is the wisest thing a person can do. Life is uncertain and the arm of flesh frail. Man's hopes and promises can be nothing more than wishful thinking. But those whose trust is in the Lord will never be confounded.
They can rest contentedly and have cause to be happy.
They are happy because their life is in the Lord's hand. Happy because they know His salvation.
God's promises cannot fail: you are glad to realise them when your trust is in the Lord. The Lord enables you to do His will, for your life to be fulfilled and for you to know His joy and peace. You are not taking liberties or being presumptuous when you rejoice in the Lord. It is the thing we naturally expect of you.