A formula, by definition is a series of inputs that result inevitably in a predictable output: A + B = C.
A pattern is a plan, based on experience and/or observation, for producing a predictable output. A pattern however, can be modified by lots of varying circumstances outside the control of the "creator" and end up producing radically different "outputs".
It is efficient to use patterns for one's life, and we use them all the time in the Church. There is no reason, for example, to burn your finger on the stove, if someone else has already had that experience and you can learn from it vicariously. Thus, following the principles articulated in the Word of Wisdom can be much safer than insisting on understanding addictions fully by becoming addicted then overcoming the addiction. Even though not all who partake become addicts (there is no firm formula for addiction to some substances), there is enough of a discernible pattern to warrant many people treating the pattern as if it was a formula.
There are issues, however, to doing that - to seeing patterns as if they were formulas:
First, to some degree, as we rely on patterns or formulas, we give up our ability to experience life personally and up close. Obviously, we want to avoid unnecessary dangers, but is it really life if we engulf ourselves completely in patterns and formulas? Maybe we risk not seeing the sunrise differently that someone else - and that would be a shame. Maybe we risk closing ourselves to the whispering of the Spirit - and that would be an even greater shame.
The second thought is that as an LDS culture, we tend to take a pattern and think of it as a formula. (In fact, this tendency is deeply rooted in the "natural (wo)man".) For example:
- Marrying a returned missionary will make you happy.
- Marrying in the temple will lead to the Celestial Kingdom.
- If you marry a returned missionary in the temple, you will be happy and end up in the celestial kingdom. (A + B = C)
Any time that we think of a pattern as an absolute formula, we are eventually BOUND to be gravely disappointed by: the returned missionary who goes 'astray' - the temple marriage that goes bad - the general unhappiness we might feel. Perhaps, some of that general unhappiness (or shattering disappointment and disillusionment) might be because we feel we haven't really experienced "our own" lives and, instead, simply lived a pattern believing it to be a formula.
I think it is important to understand the heart of what my friend said - that we can follow a general pattern (and that it's important to do so in many cases), but that it's just as important to find and follow our own paths and the promptings of the Holy Ghost that can tailor and alter a general pattern to one that is right and inspired and directed for each of us as an individual. We need to understand how few formulas there are in mortality and, instead, see and adapt patterns that have meaning to us - and that will not shatter our perspectives if they do not act as formulas that create "C" and lead us instead to "D" - or "L" or even "Z".