Some people readily succumb to temptation of a huge sum of money for certain information - especially, if such deal is promised to be confidential and will not interfere with their daily routine.
Personal contact is the second common cause.
Someone you know or a relative of yours offers a helping hand at a time of need and then, later on you are asked to return the favor by becoming a spy. This temptation is particularly strong for people who are not socially successful and lonely. People with certain weaknesses and a past to hide are easy target for spy masters. You could threaten these folks with exposure unless they did some 'harmless' job for you. You would reward him with some money and force him to do more tricky jobs.