In this sonnet, the poet Elizabeth Browning asks her lover Robert not to love her for transitory (short-lived) reasons like her beauty and gentle nature. In this connection she also mentions that he may fall in love depending on a particular thought that is compatible and complementary to hers. That compatibility may give him a sense of pleasant ease on that particular day. But according to the poet, that would be an unwise reasoning for showing his love for her, as that is not going to last long.

So, to answer your question precisely:

What provides a sense of pleasant ease?

A ‘trick of thought that falls well with mine’ or the compatibility of the thought process between the two lovers provides that sense of pleasant ease.

In this sonnet, Trick of thought can also stand for momentary fancy or rather an illusion of love. The poet explains to her lover that her transitory qualities may make him feel that he is in love with her on that particular day. And this feeling or illusion of love that he has will bring a ‘sense of pleasant ease’ only on that day. If the next day, the poet looses her superficial/transitory qualities, then her lover won’t love her and then he wouldn’t have the ‘sense of pleasant ease’.

Therefore, the sense of pleasant ease here stands for the momentary fancy, or the illusion of love that the poet’s lover may experience, if he loves her for her transitory qualities and not unconditionally.