The Samaritan leper, "realizing that he had been cured, came back praising God in a loud voice. He threw himself on his face at the feet of Jesus and spoke His praises" (Lk 17:15-16). Jesus doesn't mention this so much because He desires to be thanked. Rather, He knows how necessary it is for our salvation that we "render constant thanks; such is God's will for" us (1 Thes 5:18).
In the passage immediately preceding this incident, Jesus also sheds light on man's posture before God. Jesus tells us, "When you have done all you have been commanded to do, say, 'We are useless servants. We have done no more than our duty' " (Lk 17:10). It's part of our fallen, human nature to take God's sheer, unmerited goodness to us for granted. We can even expect God to "be grateful to" us because we serve Him (Lk 17:9).
"Jesus took the occasion to" comment on the importance of the proper relationship of man to God - that of humble, grateful praise (Lk 17:17ff). Refusing to give God thanks begins a downward spiral in our relationship with Him (Rm 1:21ff). We make a deliberate change in our posture toward God. We stand proud rather than lie prostrate. Our hearts are darkened; our minds lose wisdom and grow foolish (Rm 1:21-22). We slide into a lifestyle of sin and perversion (Rm 1:24ff), and do "not see fit to acknowledge God" (Rm 1:28), which can lead us into hell (Mt 10:32-33).
The word Eucharist means "thanksgiving." Like the healed leper, make a thankful return to God (Ps 116:12) by dedicating yourself to Him in the Mass often, even daily. Thank You, Jesus!