So, four strange parables, the first two about what God is doing in offering the gift of the kingdom (mustard seed, leaven), the second two, about how we are to seek and accept that gift (treasure, pearl). They are followed by an allegorical parable (13:47-50) that serves as an exclamation point to all four of them. Its message seems to be: better recognize the gift and work at accepting it or face the consequences.
This reminds me of the graduation ceremony in which the President of the University stood on the stage, handing out diplomas as the large group of graduates paraded by, one by one. If you were close enough, you would have heard him say to each graduate, "Congratulations" (as he handed them their diploma) and then "Keep moving" (as he shook their hand). He meant it literally—the event was on a schedule. But it's got a good metaphorical meaning, too.
"Congratulations, and keep moving." Matthew is saying the same thing as he hands his community (and us) these five parables from chapter 13.
C.H. Dodd, Parables of the Kingdom, 1935, 2nd ed., rev. (New York: Charles Scribner's Sons, 1961).
Douglas R. A. Hare, The Interpretation Commentary on the Gospel of Matthew
Alyce M. McKenzie, The Parables for Today