Why is prayer necessary for christians?

Because it is the chief part of thankfulness which God requires of us: (a) and also, because God will give his grace and Holy Spirit to those only, who with sincere desires continually ask them of him, and are thankful for them. (b)

(a) Ps.50:14 Offer unto God thanksgiving; and pay thy vows unto the most High: Ps.50:15 And call upon me in the day of trouble: I will deliver thee, and thou shalt glorify me. (b) Matt.7:7 Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you: Matt.7:8 For every one that asketh receiveth; and he that seeketh findeth; and to him that knocketh it shall be opened. Luke 11:9 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. Luke 11:10 For every one that asketh receiveth; and he that seeketh findeth; and to him that knocketh it shall be opened. Luke 11:13 If ye then, being evil, know how to give good gifts unto your children: how much more shall your heavenly Father give the Holy Spirit to them that ask him? 1 Thess.5:17 Pray without ceasing.