Given the volume of people leaving the firms for greener pastures, I started thinking about the reasoning behind why people at different levels within the firm leave. So let's give this a shot...
1) First year associate - You go, WTF is this job? You realize that this is not the career or field, let alone the job, for you. You realize your calling is nursing, teaching in an elementary school, running a store, or something on those lines. This is when you leave, or should leave.
2) Second year-third year associate - You complete two years at the firm. You cannot deal with the hours, and your main priority is a good work-life balance. You'd rather be an accountant at a company, and have your own desk and 9-5 hours. This is when you leave, or should leave.
3) First senior associate year- You now have senior year under your belt. You probably have your CPA by now. The offers are pouring in. You hate the salary you're getting paid. You went through a really difficult year and start to hate some of your managers. You want out. You get an offer paying 8-12k more, with something called bonuses given to you on a periodic basis. You don't mind working from 8:30-6:30 or so. More specialized positions like revenue recognition accountant come into play. This is when you leave.
4) Second senior associate year - You put in your time at the firm. You know you do NOT want to be a manager at the firm. You realize that if you stay one more year, you'll end up staying for two. You don't think you can deal with this kinda life anymore. Friends around you are dropping like flies. You think about leaving once a week. You go through a range of emotions, going back and forth between leaving and staying. You give interviewing a shot. You decide that if you don't like any positions out there, you're staying. A combination of a catalyst event happening at work (work till midnight for a couple days) and an intriguing job offer make you leave.
5) Third year senior associate - You're this close to becoming a manager. You are pushed to the limits at work. Your salary is absolutely ridiculous for the amount of work you do. All your friends in the private sector are making more than you doing way less work. You wonder if waiting out a few months to make manager is worth it. Your mind's telling you to wait it out since it will be worth it in the long term. But you get an offer that might be the same as when you make manager, and you do not want to let this slip by. This is when you leave.
6) Manager year (1st and 2nd) - Holy heck, what just happened. You're over-worked, frustrated, and stressed out. But you have the mgr year under your belt. There is no reason to stay, unless you don't know what to do, or you want to be in audit. Assistant controller and controller positions come pouring in. You jet.
7) Manager year (3rd-4th) - You could become a senior manager. Maybe you can wait it out and get that title. But your personal life comes into play. Marriage and kids come into play. Your significant other wants more time if you're a guy, and if you're a girl, kids make staying not worth it. You jet for a good job with decent hours so you have a life and spend time with your family.
8) Senior manager - You realize your chances of making partner are less than 20%. It'll take you time to check your ego, but it hits you finally. This is when you leave. (Except I don't get why you don't leave. I really don't. Don't you know by now you won't make partner. Maybe it's that 20% chance that makes you hang on).
9) Partner - Retire. CFO/Director position. Forced early retirement. That's it. You've hit the holy grail in the accounting field, and are set for life, so why bother leaving.