To read the texts click on the texts: 1Thess 2:1-8; Mt 23:23-26
The fourth (23:23-24) and fifth (23:25-26) woes against the Pharisees are about focussing on the insignificant matters and externals while forgetting what is significant and internal. The Pharisees were extremely particular about tithing and to ensure that they did not err in this regard, tithed even small garden vegetables used for seasoning which Matthew mentions here as mint, dill and cumin and probably in order to correspond with justice and mercy and faith.
Gnat and Camel, which the Matthean Jesus contrasts in 23:24, were the smallest and largest living things in ordinary experience. While the Matthean Jesus does not state that what the Pharisees are doing is wrong, his critique is that while focussing so much on these insignificant items, they lose sight of the larger picture.
Too much focus on the external can also lead to forgetting the internal. What is on the outside is merely a reflection of what is within.