It's because of the Security options.
Go to System Preferences... > Security & Privacy and there should be a button saying Open Anyway, under the General tab.
You can avoid doing this by changing the options under Allow apps downloaded from:, however I would recommend keeping it at the default Mac App Store and identified developers.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
