You're both right, but the reasons aren't directly related to Democracy as a government type.
When 'Democracies' had a very limited franchise, as in the pre-20th century when in most states only 'property owners' and, basically, Upper Middle Class and Above could vote, Democracies went to war as often as anyone, because the chance of getting shot at was very low for those voting for the government that started the war. Once every male in the country could vote, the people who voted for war were the ones getting killed, and they became much, much less enthusiastic about it. Even in Nazi Germany, a warped Democracy if there ever was one, with all the totalitarian trappings of controlled press and relentless pro-war propaganda, the reaction to the start of a Major War in September 1939 was noticeably negative among the population.
Which brings up the other 'real' reason nation-states don't go to war much any more. The European nation-states, which still control a huge percentage of the war fighting industry, weapons, and skills, have had two really, really bad experiences in the twentieth century of what can happen in a war, and they have too many immediate reminders to want to repeat any of that experience.
The real drawback to Democracy is not from starting a war, but as the one poster commented, staying in the war for any length of time. Any major defeat will accelerate War Weariness in a Democracy dramatically, because, in the last 200 or so years, that defeat will reach the population almost immediately in newspapers and telegraph/radio communications. Any major victory, on the other hand, IF not accompanied by major casualty lists, will reduce War Weariness.
A Democracy at war, then, becomes very Risk-Adverse: look at the discussions before D-Day of the Political Risks to attempting a major amphibious landing and having it fail with thousands of casualties - it was assumed that they would not be able to make any second attempt, because the political costs would be just too high. They also didn't dare make an attempt in 1942 or 1943, when German defenses were much, much less ready, because they could not guarantee a success - risk aversion on a grand scale.