Because thou hast been my help - Because thou hast interposed to defend me in danger. The idea is, that he had experienced the divine interposition in times of danger, and that this was a reason why he should still confide in God. The argument is, that God's mercy and favor in the past is a reason why we should confide in him in time to come.
Therefore in the shadow of thy wings will I rejoice - Under the shadow or protection of thy wings will I feel safe. See the notes at Psalm 17:8. Compare Psalm 36:7; Psalm 57:1; Psalm 61:4.

Therefore in the shadow of thy wings - I will get into the very secret of thy presence, into the holy of holies, to the mercy-seat, over which the cherubs extend their wings. If the psalmist does not allude to the overshadowing of the mercy-seat by the extended wings of the cherubim, he may have in view, as a metaphor, the young of fowls, seeking shelter, protection, and warmth under the wings of their mothers. See the same metaphor, Psalm 61:4 (note). When a bird of prey appears, the chickens will, by natural instinct, run under the wings of their mothers for protection.
The old Psalter translates, And in hiling of thi wenges I sall joy. The paraphrase is curious. "Thou art my helper, in perels; and I can joy in gode dedes in thi hiling, (covering), for I am thi bride, (bird), and if thou hil (cover) me noght, the glede (kite) will rawis me, (carry me away.")

Because thou hast been my help,.... Or, "that thou hast been my help" (s); and so the words may be considered as the subject of his meditation in the night watches, at least as a part of it; and as what gave him a great deal of pleasure to reflect upon, how the Lord had been in times past a present help to him in time of trouble;
therefore in the shadow of thy wings will I rejoice; meaning under the protecting power of God, where he knew he was safe, and therefore had reason to rejoice. The allusion is to the chirping of chickens under the wing of the hen; see Psalm 57:1. The Targum is,
"in the shadow of thy Shechinah will I rejoice;''
referring it may be to the Shechinah, or presence of God, between the cherubim, whose wings overshadowed the mercy seat.
(s) "quod", Musculus, Gejerus, Michaelis; so Ainsworth.

True Christians can, in some measure, and at some times, make use of the strong language of David, but too commonly our souls cleave to the dust. Having committed ourselves to God, we must be easy and pleased, and quiet from the fear of evil. Those that follow hard after God, would soon fail, if God's right hand did not uphold them. It is he that strengthens us and comforts us. The psalmist doubts not but that though now sowing in tears, he should reap in joy. Messiah the Prince shall rejoice in God; he is already entered into the joy set before him, and his glory will be completed at his second coming. Blessed Lord, let our desire towards thee increase every hour; let our love be always upon thee; let all our enjoyment be in thee, and all our satisfaction from thee. Be thou all in all to us while we remain in the present wilderness state, and bring us home to the everlasting enjoyment of thee for ever.