One of the basic things you need to think about when you are designing something is it's shape. What is the general outline? Is it square? Round? Oblong? A snowflake is mostly round with some points, and the little parts that make up the whole fit together like pieces of a pie with each piece of the pie being a single pattern repeat.
If you want to make a heart, how would you describe the shape? Hearts kind of look like triangles. Some are short and fat and some are tall and skinny, but all of them are wider at the top than the bottom and all of them have a point at the bottom.
If you want to start with a snowflake and turn it into a heart, what part of a round snowflake could you fit into a heart? Are there bits that hang over the edges? Are there areas you have to fill in?
Take a look at this picture. Does it give you any ideas? One of the smallest heart shapes you can make is just a simple cloverleaf surrounded by a chain. If you take a look at a lot of the heart patterns available, you'll see that may of them use a cloverleaf as their base. In a simple cloverleaf heart the central picot is often made longer to accentuate the heart shape.
Let's say you want to make a heart that's bigger than a cloverleaf. Do the circles in the picture suggest anything else? How about 3 snowflakes?
When you create a snowflake, you want a six sided shape with points at the tip of each side. For the purpose of creating a heart the points just get in the way, so the extra bits that make a round motif into a snowflake are just omitted.
Look at the area of the heart shape not covered by the circles. Notice that there are 2 small triangular areas on either side and another at the bottom. To create a heart from 3 snowflakes you need to add a little bit of tatting in between each top motif and the bottom one, and another little bit at the bottom to accentuate the heart shape.
Attaching motives together gives you the right general outline, but if you want an unmistakable heart you need to tie it all together by adding a defining row of chain.
That's what I did with this heart. I used the centre part of Snowflake 5 and omitted the outward facing rings. The 2 top motives are joined along the flat side of the hexagon shape and the bottom one joins at the point of the hexagon shape. That arrangement meant that the top motives were rounder and the bottom one had a point at the bottom.
The shape suggested a heart, but it really became a heart when the outer row of chain was added connecting to the middle of each chain on the motives and adding the cloverleaf to fill that space between the motives on each side. Since the bottom motif already had a downward point, adding a cloverleaf would have been too much. A single outward facing ring was enough to finish it off.
If the motif had been more round and not quite so pointed, a cloverleaf could have been used.
The same thing can be done with any round motif. Just select a design you like, attach 3 of them together in a triangular shape and then fill in the spaces to make your own heart.