When you’re training for a fitness goal, a workout buddy can be your best asset. She’s the one who cheers you on good days and pushes you on bad days.When you’re working to reach a financial goal, friends can be equally helpful. Peers can influence how much you save for retirement, determine when you choose to retire and even shame people into paying your taxes, studies show.People don’t change their habits when the stakes are low, said Dean Karlan, a Yale economist who has studied incentives and …