Step 1: Create a new document (either transparent of filled with white). For this example the background will be filled with white, RGB and 300 dpi 3"x3".

Step 2: To make the pattern I will use the preset shapes that come with Photoshop. For this example I will be using fill pixels set to 60% opacity. Using the shape of a heart pick a color from the swatches and start drawing them into your document. Keep your hearts on the inside of your document.

Example 1:

Above: hearts placed randomly inside a box.

Step 3: Once you have placed your hearts inside your box, you will choose Offset Filter. This is located under Filter/Other/Offset. Move your sliders around so that it looks good for you.

Example 2:

When you turn the above hearts into a pattern you will notice that there is a lot of space around the edges that does not look good. See below.

Example 3:

Step 4. After experimenting with a few of these, I discovered that if you take the pattern once it has been offset and fill in some more hearts (make sure you don't go outside the edge of your box) you will not get an invisible outline that shows up once you make the pattern. Below is example 2 with more hearts filled in.

Example 4.

Step 5: Under Edit, select Define Pattern. Do a select all and add this to your patterns. Your final pattern will fill in like below.