Sometimes marriage is good. Sometimes marriage is bad. At least half of all American marriages end in divorce. Plenty of couples had no business getting together in the first place, especially to have children (poor children). I think many of us know how badly parenthood can go.
So why do engagements receive mostly positive responses? Even the most indifferent co-worker usually offers a "Congratulations," even if they could really care less. There's huge cultural pressure to participate in, or at least not oppose, the general good cheer and optimism that result when a friend or co-worker gets engaged.
If marriage so often goes badly, why do people get so happy at the prospect of another one?