Mate, it's really hard to think without referring to a certain category of religions as this involves very different concepts of God. But this is what I think, in logic. Again I refer God as something superior.
Superior Meaning and Definition
1. (a.) Belonging to the part of an axillary flower which is toward the main stem; posterior.
2. (a.) More elevated in place or position; higher; upper; as, the superior limb of the sun; the superior part of an image.
3. (a.) More comprehensive; as a term in classification; as, a genus is superior to a species.
4. (a.) Higher in rank or office; more exalted in dignity; as, a superior officer; a superior degree of nobility.
5. (a.) Above the ovary; -- said of parts of the flower which, although normally below the ovary, adhere to it, and so appear to originate from its upper part; also of an ovary when the other floral organs are plainly below it in position, and free from it.
6. (n.) The head of a monastery, convent, abbey, or the like.
7. (a.)
Beyond the power or influence of; too great or firm to be subdued or affected by; -- with to.
8. (a.) Pointing toward the apex of the fruit; ascending; -- said of the radicle.
9. (n.)
One who is above, or surpasses, another in rank, station, office, age, ability, or merit; one who surpasses in what is desirable; as, Addison has no superior as a writer of pure English.
10. (a.)
Higher or greater in excellence; surpassing others in the greatness, or value of any quality; greater in quality or degree; as, a man of superior merit; or of superior bravery.
In other words, God as a deity -
Deity - Monotheism"In some cases, especially the monotheistic Abrahamic god or the supreme deity of henotheistic religions, the divine entity is not thought of by some believers in the same terms as deities - as a
powerful, anthropomorphic supernatural being - but rather becomes esoteric,
the reification of a philosophical category - the Ultimate, the Absolute Infinite, the Transcendent, the One, the All, Existence, or Being itself, the ground of being, the monistic substrate, etc."
Thus, as God is the Ultimate, the Absolute Infinite et cetera, he is not the same as us. He is more superior than human, means he does not have feelings like being lonely, sad, horny, constipated et cetera. Give this example a thought : if God can feel lonely, of course he is the same like us as we feel lonely too. If we can shit, of course God can shit too right? If we can die, God dies too right? OMG your God dies

Of course God don't feel lonely, as he couldn't shit or he couldn't die. He is the ultimate, the most supreme, that's why human call him God. Thus the statement of "God created us because he is lonely" does not fill in the criterias why we must call him God. And you know, if we kinda think of it, God doesn't have a son too, because God doesn't have sex, he's different and more supreme than us. No offense to Christians, but I used to question a lot of religions including mine.
Okay, lets not stray away. The main question is actually, why God created us to worship him?
Answers later. I'm trying my best to think like an atheist now LOL.
Added on November 17, 2009, 4:12 pmReferring to my previous post, let me explain the "atheistic opinion" on Why Must We Worship God.
Come to the concept of worship. In every religion, it is stated that we should worship God because of LOVE.
1. God loves us
2. We should love God too
What's the proof that God loves us?
1. The universal, cliched opinion : He created the universe for us, we should be grateful for that
2. Even though a lot of us don't believe in him, he still provide oxygen, a place to live in, not much physical threats like once-you-dont-pray-then-say-goodbye-to-life
And did it ever occured to us that we should love God back too? Who are we to expect all comforts from someone who loves us without us loving back in return? God isn't being tyrannical. He is being loving, he loves us too much and he wanted to be loved back. Sounds so humanistic for you? Let me tell you something, every religion stated that God loves us. That does not mean that the love is to spoil. There is a proverb in Bahasa Melayu, heck I can't find the equivalent in English "sayang anak tangan tangankan"。 When you love your kid, be stern with him, to teach him to be human. Let us balance this proverb in this case. God loves us too much, that he taught us to love him back, even threaten us with hell to stop us to find other Gods. This is not arrogance, this is pure Godly love. Even though a lot of human does not believe in him, he still pampers them with life, just that he showed a lot of signs and threats (hell, doom etc), so that we don't stray away from loving him. So different from humanly love.
Then, why do he created us in the first place? I'd say that he created us to find is there any of us that will love him back. This is why the concept of heaven/ nirvana is there, that's the reward for people who loved him back. How to love him back? Worship him, simple. It's like showing your love to a boyfriend, just that you don't show your boobs to prove your love, you show your prayer to love him.
And then, why, after all the worshipping, God still give us hard times like recession, tsunami, high crime rates etc?
1. This is because God wants us to love him more. When we underwent hard times in this world, we'll pray more. Prayers works spiritually, it calms you down, and you feel supported by 'something' (you'd know this if you really meditated before). The 'something' is God. God loves to be loved. I know this rationale is very abstract, but then theology itself is one abstract thing that it's really hard to think the other way round and try to answer the opposite

2. Those crimes, are caused by people who does not obey God. It's like mothering a kid. A mother set up rules and regulations for a child, e.g don't cross the road before looking left and right. But then when the child disobeys, fatal endings. It's the same on this case. God set up religion and regulations for us e.g be honest, don't steal etc. But then when we did the opposite, crime rates soared high and the world is not a better place for us.
So friends, these are what I think about why must we worship God. Correct me if you guys saw any loopholes, I know this is waaay abstract, it's not an easy question too, ya know

ok.. we got a problem.. no matter how we see it god still stuck with concept in religion...
let`s do this... erase the meaning of god from religion perspective... and now GOD is just an empty word with no meaning that will used to define existencism....
and look from science view big bang etc... can the word god fit in?