The Beginning of David influence takes place in the Valley of Elah when he was sent by Jesse to bring provisions to his bretheren.  While speaking with his brothers, Goliath the champion and warrior of the Philistinian army boasted against Israel.  David inquired after the people regarding what type of reward would a man get for killing the Philistine.  The children of Israel responded that whosoever killed the Philistine would be exalted, given riches, and he would become affiliated through marriage to Saul's daughter and his family would become free. 

Now Jesse is stated to be of the house of Epthratha, the woman who was taken by Caleb, the son of Hezron to become his wife.  And Ephratha was the mother of Hur, known as a prince of Midian and the forefather of Bezaleel the artificer of the ark of the covenant.  Now the point to be made is that Ephratha may be another name for 'Ephah' a daughter of Midian and a concubine.  Which would mean that Jesse was of the captives in Israel.

So the information provided in 1 Samuel 17 points out that the main reason for David's challenge against Goliath being ambition.  And so it was that
David did slay the Philistine, and afterward he became a man of great wealth and reputation.  Now the people desired to make David a kinsman to Saul by way of marriage and fulfilling their vow that he should take one of Saul's daughters as a wife.  David being resistent at first was commanded to go forth to battle against the Philistines and if he should bring back a hundred foreskins of the Philistines (indicating forced circumcision) that he should be given Merab, the eldest daughter of Saul as a wife.  Apparently it was not the desire of Saul to give Merab unto David but that he should fall in battle against the Philistines, and so he gave Merab to Adriel the Gileadite.  However, when Saul saw that Michal loved David he gave her unto him for a wife. 

Now before all these events, David was encouraged by the acts of Samuel who was sent that he might elect and raise up one of the sons of Jesse as an adversary to Saul.  And Saul understanding that David was chosen to become the ruler over Israel distrusted David.  Yet, the distrust was not consummated until we learn of the following events involving the praising of David before the sight of Saul.  Now it states that the women song songs diminishing Saul and exalting David, and so it was for this reason that Saul despised David.  However, I believe the hatred went a lot further then just the songs sung by the women.  Now Saul was said to have had a wife named Ahinoam, the daughter of Ahimaaz, the son of Ahimelech, the son of Abiathar and descendant of Ithamar son of Aaron and Elisabeth.  The same name of the woman whom David is said to have taken for his wife while he fled from Saul amongst the Philistines. 

I don't believe that Saul's wife and David's wife both just happen to possess the same name.  In fact, I believe that Ahinoam was in love with David, and later fled with him to the land of the Philistines.  My view is supported when you read 1 Samuel 20:30 when Saul in a fit a fury spoke against Jonathan's support of David  saying, "Thou son of the perverse rebellious woman, do not I know that thou hast chosen the son of Jesse to thine own confusion, and unto the confusion of thy mother's nakedness?" And I believe this was the reason why David went to Ahimelech the grandfather of Ahinoam and the priest amongst the Gibeonites, and why when Saul learned of his whereabouts he had Ahimelech slain.  Now the verses clearly reveal that Ahimelech was suspicious of David's visit and was affraid when he saw he came alone without the company of men  he apparently was known to govern.  And because he knew not that David rebelled against Saul, Ahimelech came to his unfortunate end in Nob, after having supported David.  This act against the house of Ahimelech would be lead to an conspiracy against the remaining descendants of Saul later.  I also believe that this event was known and was the reason why Nabal as he is called in the bible spoke such harsh words against David when he sent messengers for support.

These events may have been predestined when Samuel chose David prophesying that David should possess Saul's kingdom and all belonging to Saul possibly even his wife.  And just as this act was seen as treason when Absalom was advised to lay with David's wives whom he left behind, it is reasonable to say it was seen as treason in the days of Saul.  And such an act would have forced Saul to pursue David with the intent of destroying him.  It is also interesting to learn that Achish, the king of Gath belonging to the Philistines supported David after he slew Goliath.  And it is strange how certain verses found in 1 Sam. 21 states that when David fled from Saul unto his enemy king Achish that the princes revealed his identity and David played mad to escape what apparently was a dangerous situation.  Yet, David managed to obtain the trust of this king as we learn that he was a trusted officer of Achish's court and was given Ziklag as a dwelling place.

The Deception of David an officer of Achish:

When David fled certain men of Israel went into exile with him.