Why do Catholics have to go to Mass to be with God?

“Love the Lord with all your heart, and with all your soul,
and with all your MIND”--Matt 22:37

 
Question:  Why do Catholics have to go to Mass to be with God?  I can be with God whenever I see a beautiful sunset or go on a hike in the mountains.
 
 
In one sense, it's true that since God is everywhere--ever present, ever loving--He is absolutely present in a beautiful sunset.  He is visible in all creation.  However, He is only physically, truly and substantially present at Mass, in the Eucharist.  There He is present:  Body, Blood, Soul and Divinity.
 
It's like the difference between saying, "My family is always with me, even when I'm at work, or  when I'm out of town.  I can feel their presence in my heart at all times."  versus  "My family is with me, physically and substantially, when we are together at home."
 
From This Rock magazine, 2007:
(Permission granted)
 
To consider just one example, the Romantics of the early nineteenth century (and their heirs down to the present day), held that God is to be found in nature and personal experience, not in a visible Church or in memorized prayers and revealed dogmas. God, they said, is immanent in his creation; one need only rouse up the innate spiritual sense to discover him. This "spiritual but not religious" attitude, still prevalent today, assures the individual that his subjective perceptions or feelings are the only guidance he needs to probe all matters moral, theological, or metaphysical. "Preserve a pure, childlike sensibility, and follow without question your own inner voice, for it is the divine in us, and does not lead us astray," wrote German Romantic painter Caspar David Friedrich (1774-1840) (qtd. in Schmied,Wieland, .aspar David Friedrich, 44-45)...

[While] Catholics believe that man stands at the summit of creation, "a little less than the angels," even though he is but dust and ashes, infinitesimal before the majesty of God. We too look for God, and aspire to be united with him—through the graces of the Holy Spirit, prayer, and the sacraments, and yes, the contemplation of nature and art. Further, while we believe that God can and does reveal himself in obvious, tangible ways, we say that he also "hides" sacramentally and mystically within his material creation: "The world is charged with the grandeur of God," in G.M. Hopkins’s words. Thus, those who find it spiritually beneficial to ascend a mountain in the wilderness should not be denied the opportunity. But we are still called to worship God in church...

The same allusive natural spirituality that appealed to the Romantics could be used to direct contemporary unchurched seekers toward the fullness of the faith. Show them how their yearning for union with the divine spirit they obscurely sense in nature can be fully satisfied by communion with Christ. We might say..."Come with me into church, and you will meet God on the altar." (bold mine)
 
The "spiritual but not religious" paradigm is a way, in my opinion, for those who want the good "feelings" that come from acknowledging there is a God, but don't want to obligate themselves to change their lives or behavior. 

For more in-depth study visit these websites:

Catholic Bible online

Catechism of the Catholic Church online

  "Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect" - 1 Peter 3:15