Not for what He has done, but simply for Who He Is. Here a beautiful excerpt from Psalm 103:
1Bless the LORD, O my soul: and all that is within me, bless his holy name.
2Bless the LORD, O my soul, and forget not all his benefits:
3Who forgiveth all thine iniquities; who healeth all thy diseases;
4Who redeemeth thy life from destruction; who crowneth thee with lovingkindness and tender mercies;
5Who satisfieth thy mouth with good things; so that thy youth is renewed like the eagle’s. […]
10He hath not dealt with us after our sins; nor rewarded us according to our iniquities.
11For as the heaven is high above the earth, so great is his mercy toward them that fear him.
12As far as the east is from the west, so far hath he removed our transgressions from us. […]
20Bless the LORD, ye his angels, that excel in strength, that do his commandments, hearkening unto the voice of his word.
21Bless ye the LORD, all ye his hosts; ye ministers of his, that do his pleasure.
22Bless the LORD, all his works in all places of his dominion: bless the LORD, O my soul.