I think dwarf happiness is best achieved by a confluence of several organizational things. Here are my thoughts:
- Make them join the military early so they form their social bonds to fellow soldiers. Otherwise, after they have been slacking off in the fortress for three years as your accasional hauler, they will complain about the draft and not seeing their loved ones when on duty.
- It does help to raise Discipline if you let them see corpses frequently, e.g. let them frequently walk by a corpse stockpile. After a while they won't feel anything when they see somebody die.
- Let them fight often (arena fights) or spar often. It's very good if you read in their happy thought about having had a satisfying sparring session.
- Keep them out. It's not nice when your squad is cave-adapted. The moment they get out they are nauseated by the sun which is an unhappy thought.
- Give them some off-time. I usually have 80% of the squad members train, patrol, or station for 2-3 months, then giving them one month off.
- Give them their favourite food and drinks. If you love micromanagement, you can make stockpile of your soldiers favourite foods and drinks in a dining hall or tavern close to your barracks. Otherwise just rely on food and drink variety. Try to buy lots of different food and drink items from your traders and hope that your dwarves fill find their favourites.
- Assign your soldiers pets, but pasture them in the barracks or in the soldiers' quarters. Do not allow pets join your soldiers in battles. Soldiers will get happy thoughts when they see their pets.
- Have temples where your religious soldiers can commune with their deities. Masterwork decorated temple equipment helps.
- Have a waterwall in the barracks, tavern, or any place which the soldiers frequent. They will be relieved being close to a waterfall.
- Have a high quality hospital. Masterwork beds, masterwork traction benches (with masterwork mechanisms, masterwork ropes/chains), decorated floor, nice table, chair, chest adjacent to every hospital bed, preferably decorated. This will give your patients happy thoughts while they recover from wounds.
- Cherry-pick doctors and nurses who are suitable for their roles, i.e. have a high empathy, love to help others, console others.
- For ultimate happiness, have the furniture in the barracks and in the soldiers' private quarters made of their favourite material.
- Set child cap to 0 to prevent soldiers from giving births on the battlefield, or don't draft women into your military.
- Care for the soldiers fitness. Strong soldiers attack and defend better, wear heavy armor with ease. Run them through a treadmill training program, i.e. let them often operate a water pump (even when not actually pumping water). Your soldier will grow muscles.
- Have good performers in the tavern your soldiers frequent.
- Have something nice for them to read in your library.