mingle, mixing, association, coming & going
交 centers on things crossing or interchanging: people mingling, ideas mixing, or traffic coming and going. The association sense is a natural extension of this crossing interaction.
交 is a pictograph of a person with crossed legs, representing the idea of crossing or interchange. The modern form retains the crossed-stroke silhouette.
The top lid 亠 and the crossed legs of 父 combine to show two things crossing over each other, like people mingling or paths intersecting.
For コウ, imagine a busy intersection where cars cross and mingle, and a traffic cop (コウ) directs the coming and going: cop -> コウ.
exchange; interchange; switching; reciprocity; barter; substitution; replacement; clearing (of checks, cheques)
traffic; transportation
negotiations; bargaining; discussions; talks
exchange (e.g. cultural); interchange; interaction; mingling; mixing; coming together
diplomacy
change; alternation; rotation; replacement; substitution; shift; relief; taking turns
company; friendship; association; society; acquaintance
crossing; intersection
to exchange (messages, greetings, arguments, etc.)
delivery; issuance; handing over; granting
social life; social intercourse
sexual intercourse; sex; coitus
to mix; to combine; to include
alternate; alternating
diplomatic relations
crossing; intersection
war; battle; hostilities
police box; koban; small neighborhood police station
telecommunications; correspondence
to cross; to intersect; to join; to meet
trade; commerce
friend; companion; acquaintance
to fly about; to flutter about; to flit about; to fly past each other
intimacy; friendship; friendly relations
mixture; blending; complication; crossing; intersecting; interlacing
symphony
breaking off a relationship; permanent breach of friendship; rupture
mixed with; mingled with; interspersed with; dashed with; sprinkled with
traffic accident
(means of) transportation; transport; transportation facilities; transportation system; transit system
transportation network; transport network; traffic network
traveling expenses; travelling expenses; carfare
entertainment expenses
symphony orchestra
diplomat
paid dating (esp. with an underage girl; oft. involving selling of sex); compensated dating
mutual exchange
sociable