If we keep longing for what we once had (those leeks and onions by the Nile--ick), we will never fully appreciate and receive what God has for us now (manna!).
Something else I see here is the need to trust God daily. The Israelites couldn't accumulate extra manna for the next day. They had to trust that, each morning, it would fall from the sky, and they would have exactly what they needed.
Oh, to have that complete trust in God's love and provision, for physical, spiritual, financial, and emotional needs. Never leaving us in want. Never running short. Always exactly what we need--daily.
Trust and thankfulness go hand-in-hand.