The impulse to be overly nice to tenants is one of the most common mistakes made by those who are relatively new to the property management business. It's a completely understandable impulse; nobody wants to be seen as the bad guy, and as the old adage says, you catch more flies with honey than with vinegar.
But as a seasoned property management pro will tell you, your tenants are not your friends. They are your business associates. Unless a preexisting relationship exists between you and your tenants (and in which case, it may be better for everyone involved if they rent elsewhere), don't try to cultivate a friendship with them.
Those landlords who have been in the business for a few years sometimes experience the opposite impulse: they may not be nice enough. This, too, is a perfectly reasonable reaction to the experiences that many mid-career landlords have gone through. After you've dealt with years of late payments, non-stop repairs and requests, damaged properties, duplicity, dishonesty, evictions, and possibly even a court case or two, you're likely to look at property management from a slightly more cynical perspective.
To a degree, adopting a more slightly more skeptical attitude towards landlording can be helpful. At the very least, it will probably help you make better decisions during the tenant screening and selection process. But it's important to make sure that you don't let yourself become jaded, distrustful, or just downright mean. Nothing sours the landlord-tenant relationship like unpleasantness.
So which is the best approach to take? Should you be the Nice Landlord or the Mean Landlord? Well, like so many other things in life, the best approach to take in property management is somewhere right in the middle. Carve a path between mean and nice and stick to it. Cultivate an air of detached, professional courtesy in all of your dealings with tenants. This is a business relationship, and you should always act accordingly.
Even if the business relationship goes sour as a result of late payments or damage to the property, it's crucial that you stick to your script of pleasant but detached professional courtesy. If you respond to an adverse situation with too much emotion, their response is likely to be overly emotional, as well, which is likely only to make the situation deteriorate further.
For example, if you have initiated collections on an account with a balance due for late rent, you can keep your tone calm, rational, and professional, even while you escalate the rhetoric you are using in the collections process. This can be achieved by focusing on the increasing severity of the consequences that will occur if the tenant continues to withhold payment. It's much more effective to ratchet up the rhetoric with facts, rather than emotion.
So, the answer to the perennial question of whether you should play the "good cop" or the "bad cop" in your landlord-tenant relationship? None of the above. Your tenants are neither your friends nor your enemies -- think of them as business associates, and treat them accordingly. Your best bet is to strike a neutral tone of detached professional courtesy, circumventing the question of "nice or mean" altogether.