There can be a lot of misc. things that can make dwarves unahppy (some of those things you wouldnt see in the first year) but off the top of my head they are:
Lack of Soap
Lack of clothing (dwarves are shy about being naked around eachother)
and the standard things everyone's mentioned: Food, drink, beds, nobles/officers need their own blinged out rooms assigned to them.
A varriety of different food and drink types helps too.
Basically you want to think about it as fewer unhappy thoughts and more happy thoughts. Amenities bring happy thoughts, lack thereof brings unhappy thoughts. But you can combat unhappyness by providing things that make them exceedingly happy and you can encorporate this into your fortress design.
For example, placing a very high value item in a high traffic area will give dwarves happy thoughts when they walk by. you can see this in their info if you view it, it will say "Admired a very fine statue recently" So if theyre sad that they dont have a proper bed to sleep on, you can offset that with some other happy thought.
Basically, look into making a legendary meeting hall. This will solve all your problems. Essentially that means incrasing the value of your meeting hall as much as possible. This can be done easily with fine encrusted tables, statues and even a non-functional well with high value components.
Give it a shot and let us know how it goes
EDIT: I forgot to mention, losing a friend or loved one can make the entire fort lose their SH*T, so take care of your dwarves to ensure nobody has any unforseen fun