You devoted yourself to the firm and worked really hard. But your boss doesn't necessarily appreciate it. He may be nice to you for a while and you thought you had a good employer-employee relationship, but in the end it's a business. All he cares about is his own money. He won't be grateful for what you did.
This is how I felt after I left my first employer. It didn't end ugly. But I won't be so naïve ever again.
It's always better to work for a bigger company with formal procedures.