Lady Macbeth fears that her husband has too much humanity in the sense of "compassion characteristic of humane persons" (Macbeth, edited by A. R. Braunmuller, 1997).
Braunmuller points out that the First Folio had "humane" instead of "human" and that
'humane' (= gentle, compassionate) was not distinguished orthographically from 'human' before 1700 (...),
'kindness' principally means 'kinship', but also connotes 'category' ('kind' = classification, group)
and 'naturalness' ('kind' = nature).
Macbeth drank in this milk from his mother and it binds him "to the social order of Man" (Macbeth, edited by G. K. Hunter, 1967).
The association between milk and some type of weakness can also be found elsewhere in Shakespeare. In King Lear, Act 1, scene 4,
Gonerill says to her husband, the Duke of Albany:
No, no, my lord,
This milky gentleness and course of yours (...)
And in Act 4, scene 2, she calls Albany
In Gonerill's eyes, Albany lacks blood in his liver; instead, his liver contains too much milk.
And in Timon of Athens, Act III, scene 1, where Flaminius asks,
Has friendship such a faint and milky heart
It turns in less than two nights?
Here, milky can be understood as "weak, timorous" (Timon of Athens, edited by G. R. Hibbard, 1970).
Lady Macbeth uses the word "milk" figuratively;
she thinks her husband is too "humane" (in more than one sense) or possibly to weak (i.e. insufficiently courageous or determined)
to take the shortest route ("nearest way") to what has been promised to him, i.e. kingship.