The lords of the Philistines came up to her, and said to her, Entice him, and see in which his great strength lies, and by what means we may prevail against him, that we may bind him to afflict him: and we will each give you of us eleven hundred [pieces] of silver.
The reason Samson lost his strength was only secondarily that he allowed Delilah to cut his hair. The real reason was that "the Lord had departed from him" (Judges 16:20). When God’s Spirit departed from someone under the Old Covenant, the results were disastrous (cf. 1 Samuel 16:14; Psalms 51:11).