As, midway in Psalms 27:1-14; the tone changed from jubilation to humble entreaty, so, midway in the present psalm, there is a change from plaintive and humble entreaty to rejoicing and thanksgiving.
The cause of the change would seem to have been a confident assurance, arising out of the very act of prayer, that the prayer is heard and granted, so that the happy results prayed for are certain to follow.
Such an assurance is certainly not attained by all those whose supplications are earnest and devout; but David appears to have enjoyed it not infrequently (see above, Psalms 6:8-10; Psalms 7:17, etc.).