Ramban there offers a couple of reasons:
Leah's eyes were weak (29:17), so the sun would be harmful to her.
Precisely because she was older, so there was more of a concern that the shepherds would take inappropriate liberties with her.
The root in Hebrew of the verb to kiss is נשק. This root has several different meanings which can be found in the Jastrow dictionary, such as:
1) to kiss (with the mouth)
2) to equip, meaning to supply or furnish or provide for
3) to cleanse or make ritually fit
This last meaning also has a connotation of immersing in water like in the case of a mikvah ...