“No! is always easier to say than Yes.”

Siempre me fue más fácil amar que elogiar.
Voces (1943)

This seems to be derived from a statement attributed to Rear Admiral Grace Hopper, and which she regularly used in her public addresses: "It's easier to ask forgiveness than it is to get permission."
Vorkosigan Saga, A Civil Campaign (1999)

Source: Pushing Ice (2005), Chapter 14 (p. 233)

Knox College commencement address http://www.knox.edu/colbert.xml (3 June 2006)
Context: Don’t be afraid to be a fool. Remember, you cannot be both young and wise. Young people who pretend to be wise to the ways of the world are mostly just cynics. Cynicism masquerades as wisdom, but it is the farthest thing from it. Because cynics don’t learn anything. Because cynicism is a self-imposed blindness, a rejection of the world because we are afraid it will hurt us or disappoint us. Cynics always say no. But saying yes begins things. Saying yes is how things grow. Saying yes leads to knowledge. "Yes" is for young people. So for as long as you have the strength to, say yes.

Becoming Light: Poems New and Selected (1991)

Manuscript Found in Accra (2012), Solitude

Quoted in: Phyllis Bottome, Alfred Adler: Apostle of Freedom (1939), ch. 5
Problems of Neurosis: A Book of Case Histories (1929)

September 17, 1773
The Journal of a Tour to the Hebrides (1785)

Source: The Last Little Blue Envelope