Quintilian 6.3.7-9 (tr. Donald A. Russell, with his footnote):
 Though many have tried, I do not think anyone gives a satisfactory account of the causes of laughter, which is stimulated not only by certain actions or words, but sometimes also just by physical contact. Again, there is no one principle by which laughter is aroused; we laugh not only at acute or witty sayings and actions, but at stupid, angry, or frightened ones. There is thus an ambivalence about it: laughter is not far from derision.  As Cicero says,4 it has its basis in a certain deformity and ugliness. Pointing out these in others is called "urbanity"; when it rebounds upon the speaker, it is called foolishness.
Now, though laughter may seem to be a trivial matter, aroused often by buffoons (scurrae), actors of farce, or indeed fools, it nevertheless possesses perhaps the most commanding and irresistible force of all.  It often breaks out against our will, and not only forces the face and voice to confess it, but convulses the whole body with its violence.
4 De oratore 2.236.
 neque enim ab ullo satis explicari puto, licet multi temptaverint, unde risus, qui non solum facto aliquo dictove, sed interdum quodam etiam corporis tactu lacessitur. praeterea non una ratione moveri solet: neque enim acute tantum ac venuste, sed stulte iracunde timide dicta ac facta ridentur, ideoque anceps eius rei ratio est, quod a derisu non procul abest risus.  habet enim, ut Cicero dicit, sedem in deformitate aliqua et turpitudine: quae cum in aliis demonstrantur, urbanitas, cum in ipsos dicentis reccidunt, stultitia vocatur.
cum videatur autem res levis, et quae a scurris, mimis, insipientibus denique saepe moveatur, tamen habet vim nescio an imperiosissimam et cui repugnari minime potest.  erumpit etiam invitis saepe, nec vultus modo ac vocis exprimit confessionem, sed totum corpus vi sua concutit.