See this explained in the notes at Matthew 7:7-11.

And (or, therefore) I say unto you, Ask - Be importunate with God, not so much to prevail on him to save you, as to get yourselves brought into a proper disposition to receive that mercy which he is ever disposed to give. He who is not importunate for the salvation of his soul does not feel the need of being saved; and were God to communicate his mercy to such they could not be expected to be grateful for it, as favors are only prized and esteemed in proportion to the sense men have of their necessity and importance. See this subject explained Matthew 7:7, Matthew 7:8 (note).

And I say unto you, ask, and it shall be given you,.... This is said by Christ, to encourage to prayer, and importunity in it; that if any one asks of God, in the name of Christ, and in faith, whether it be bread for the body, or food for the soul; or any blessing whatever, whether temporal or spiritual, it shall be given; not according to their deserts, but according to the riches of the grace of God; who is rich unto all that call upon him, in sincerity and truth:

seek, and ye shall find: whether it be Christ, the pearl of great price, or God in Christ; or particularly, pardoning grace and mercy through Christ, or the knowledge of divine things; and both grace here, and glory hereafter, as men seek for hidden treasure; such shall not lose their labour, but shall enjoy all these valuable things, and whatever they are by prayer, and in the use of other means, seeking after:

knock, and it shall be opened to you; the door of mercy with God; the door of fellowship with Christ; the door of the Gospel, and the mysteries of it and of the Gospel dispensation and church state, into which is admission, to all that seek; and the door of heaven, into which there is entrance by the blood of Jesus: the several phrases denote prayer, the continuance of it, and importunity in it; See Gill on Matthew 7:7

Ask (αἰτεῖτε)

The word for the asking of an inferior (Acts 12:20; Acts 3:2); and hence of man from God (Matthew 7:7; James 1:5). Christ never uses the word of his own asking from the Father, but always ἐρωτῶ, as asking on equal terms. Martha shows her low conception of his person when she uses the term of his asking God (John 11:22).

Ask, seek, knock

"The three repetitions of the command are more than mere repetitions; since to seek is more than to ask, and to knock than to seek" (Trench, "Parables").

And I say unto you, Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you.

11:9-13 Ask, and it shall be given. See notes on Mt 7:7-11. These verses are a part of the Sermon on the Mount. It is probable that Luke, who does not give a chronological history, incorporates them here, though not the time when they were spoken. The same may be said of various utterances given by Luke.

11:9 Mt 7:7.

9-13. (See on [1634]Mt 7:7-11.)

11:5-13 Christ encourages fervency and constancy in prayer. We must come for what we need, as a man does to his neighbour or friend, who is kind to him. We must come for bread; for that which is needful. If God does not answer our prayers speedily, yet he will in due time, if we continue to pray. Observe what to pray for; we must ask for the Holy Spirit, not only as necessary in order to our praying well, but as all spiritual blessings are included in that one. For by the influences of the Holy Spirit we are brought to know God and ourselves, to repent, believe in, and love Christ, and so are made comfortable in this world, and meet for happiness in the next. All these blessings our heavenly Father is more ready to bestow on every one that asks for them, than an indulgent parent is to give food to a hungry child. And this is the advantage of the prayer of faith, that it quiets and establishes the heart in God.