"why are we assuming that alien life will have the same biochemistry as life on earth"

There are some assumptions made in this issue, but the biology of the purported aliens isn't one of them. The issue is whether there are other intelligent beings but the nature of the being is subordinate to the intelligence.

But we are assuming some things about the being. First and foremost is that, if there was such a civilisation, that it would be curious about the universe and would be expansionary.

What we know from our own experience is that we have advanced to our present state due to our curiosity, desire to learn, and ability to conceive and then implement a better existence. We also know that those civilisations which have previously existed on earth but became insular and incurious about the outside world, collapsed. Civilisation can't stand still...it advances or declines.

So if a civilisation did get to our level, its unlikely that it would just decide that enough was enough and that it should just sit back for the next 200 million years and enjoy itself. At the very least some individuals would seek new understandings and new opportunities and thus strike out in search of the other.

So the main and possibly most problematic assumption is that an advanced civilisation would have the same type of desire to create and learn and explore as we have. Since we don't see such a thing, we can assume it doesn't exist.

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SR,

"What is your assessment of the likelihood of life on other planets?"

Likelihood of life somewhere is high.
But each stage after that becomes less likely.

The chances of finding some form that has evolved to the increasingly complex forms grows increasingly small. So the chances of life are high, the chances of multi-celled life less so etc. Finally, the chances of getting to something equivalent to a primate is vanishingly small. And I've have been for a long while of the view that nothing exists in our galaxy that has reached the level of the human brain.

Dear mhaze,

Come on mate that is just the skeptic in you protecting you from disappointment.

I'm sure there must have been a little excitement when Oumuamua was first spotted. Echoes of Rama. 10 times longer than its width.

http://www.theguardian.com/science/2018/jun/27/scientists-solve-mystery-of-interstellar-object-oumuamua

There are still a heap of questions unanswered about this object (especially its unexplained acceleration) much less about the rest of the galaxy.

//perhaps the issue isn't about biochemistry but more about the complexity of the coding. How would the DNA...//

Carbon chauvinist.
Posted by Toni Lavis

Answer-
That was the point that it was the information that is encoded in carbon is important not the carbon itself. Yes this information could be encoded in silicon. It doesn't even have to be quarternary it could be binary. Are you chauvinist against carbon- perhaps even worse-a revisionist. And how would you prove a silicon based lifeform is alive?

"Any sufficiently advanced act of benevolence is indistinguishable from malevolence- From Clarke's Third Law"